diff --git a/docs/src/dataio_3_migration.rst b/docs/src/dataio_3_migration.rst
index 71bf19a57..d6e634e82 100644
--- a/docs/src/dataio_3_migration.rst
+++ b/docs/src/dataio_3_migration.rst
@@ -91,6 +91,7 @@ safely be removed if present in the code.
  * ``ExportData.legacy_time_format`` 
  * ``ExportData.table_include_index`` 
  * ``ExportData.verifyfolder`` 
+ * ``ExportData.meta_format`` 
 
 
 .. code-block:: python
@@ -309,3 +310,6 @@ Changes to method arguments
 Deprecated methods
  - The ``generate_aggregation_metadata()`` method is deprecated. Replace it with the identical 
    ``generate_metadata()`` method instead.
+
+Deprecated class variables 
+ * ``AggregatedData.meta_format`` - metadata will always be exported in yaml format
diff --git a/src/fmu/dataio/_utils.py b/src/fmu/dataio/_utils.py
index bff9131dc..cbb0bf06d 100644
--- a/src/fmu/dataio/_utils.py
+++ b/src/fmu/dataio/_utils.py
@@ -11,7 +11,7 @@
 from io import BufferedIOBase, BytesIO
 from pathlib import Path
 from tempfile import NamedTemporaryFile
-from typing import Any, Final, Literal
+from typing import Any, Final
 
 import numpy as np
 import pandas as pd
@@ -52,35 +52,20 @@ def dataio_examples() -> bool:
     return "RUN_DATAIO_EXAMPLES" in os.environ
 
 
-def export_metadata_file(
-    file: Path,
-    metadata: dict,
-    savefmt: Literal["yaml", "json"] = "yaml",
-) -> None:
+def export_metadata_file(file: Path, metadata: dict) -> None:
     """Export genericly and ordered to the complementary metadata file."""
     if not metadata:
         raise RuntimeError(
             "Export of metadata was requested, but no metadata are present."
         )
 
-    if savefmt == "yaml":
-        with open(file, "w", encoding="utf8") as stream:
-            stream.write(
-                yaml.safe_dump(
-                    metadata,
-                    allow_unicode=True,
-                )
-            )
-    else:
-        with open(file.replace(file.with_suffix(".json")), "w") as stream:
-            stream.write(
-                json.dumps(
-                    metadata,
-                    default=str,
-                    indent=2,
-                    ensure_ascii=False,
-                )
+    with open(file, "w", encoding="utf8") as stream:
+        stream.write(
+            yaml.safe_dump(
+                metadata,
+                allow_unicode=True,
             )
+        )
 
     logger.info("Yaml file on: %s", file)
 
diff --git a/src/fmu/dataio/aggregation.py b/src/fmu/dataio/aggregation.py
index afc757456..ad64b8d94 100644
--- a/src/fmu/dataio/aggregation.py
+++ b/src/fmu/dataio/aggregation.py
@@ -54,7 +54,7 @@ class AggregatedData:
     """
 
     # class variable(s)
-    meta_format: ClassVar[Literal["yaml", "json"]] = "yaml"
+    meta_format: ClassVar[Optional[Literal["yaml", "json"]]] = None  # deprecated
 
     # instance
     aggregation_id: Optional[str] = None
@@ -77,6 +77,12 @@ def __post_init__(self) -> None:
                 "manner instead.",
                 UserWarning,
             )
+        if self.meta_format:
+            warnings.warn(
+                "The 'meta_format' option is deprecated and should be removed. "
+                "Metadata will only be exported in yaml format.",
+                UserWarning,
+            )
 
     @staticmethod
     def _generate_aggr_uuid(uuids: list[str]) -> str:
@@ -397,7 +403,7 @@ def export(self, obj: types.Inferrable, **kwargs: object) -> str:
         logger.info("Export to file and export metadata file.")
         _utils.export_file(obj, outfile)
 
-        _utils.export_metadata_file(metafile, metadata, savefmt=self.meta_format)
+        _utils.export_metadata_file(metafile, metadata)
         logger.info("Actual file is:   %s", outfile)
         logger.info("Metadata file is: %s", metafile)
 
diff --git a/src/fmu/dataio/case.py b/src/fmu/dataio/case.py
index 5251be398..039343945 100644
--- a/src/fmu/dataio/case.py
+++ b/src/fmu/dataio/case.py
@@ -5,7 +5,7 @@
 import warnings
 from dataclasses import dataclass, field
 from pathlib import Path
-from typing import ClassVar, Final, Literal, Optional, Union
+from typing import Final, Optional, Union
 
 from pydantic import ValidationError
 
@@ -45,9 +45,6 @@ class CreateCaseMetadata:  # pylint: disable=too-few-public-methods
         description (Optional): Description text as string or list of strings.
     """
 
-    # class variables
-    meta_format: ClassVar[Literal["yaml", "json"]] = "yaml"
-
     config: dict
     rootfolder: str | Path
     casename: str
@@ -149,8 +146,6 @@ def export(self) -> str:
             Full path of metadata file.
         """
         if self.generate_metadata():
-            _utils.export_metadata_file(
-                self._metafile, self._metadata, savefmt=self.meta_format
-            )
+            _utils.export_metadata_file(self._metafile, self._metadata)
             logger.info("METAFILE %s", self._metafile)
         return str(self._metafile)
diff --git a/src/fmu/dataio/dataio.py b/src/fmu/dataio/dataio.py
index cb9aca795..7168cf8a9 100644
--- a/src/fmu/dataio/dataio.py
+++ b/src/fmu/dataio/dataio.py
@@ -363,7 +363,7 @@ class ExportData:
     grid_fformat: ClassVar[str] = "roff"
     include_ertjobs: ClassVar[bool] = False  # deprecated
     legacy_time_format: ClassVar[bool] = False  # deprecated
-    meta_format: ClassVar[Literal["yaml", "json"]] = "yaml"
+    meta_format: ClassVar[Optional[Literal["yaml", "json"]]] = None  # deprecated
     polygons_fformat: ClassVar[str] = "csv"  # or use "csv|xtgeo"
     points_fformat: ClassVar[str] = "csv"  # or use "csv|xtgeo"
     surface_fformat: ClassVar[str] = "irap_binary"
@@ -651,6 +651,12 @@ def _show_deprecations_or_notimplemented(self) -> None:
                 "key instead to set the reference for the given 'vertical_domain'.",
                 UserWarning,
             )
+        if self.meta_format:
+            warn(
+                "The 'meta_format' option is deprecated and should be removed. "
+                "Metadata will only be exported in yaml format.",
+                UserWarning,
+            )
 
     def _validate_and_establish_fmucontext(self) -> None:
         """
@@ -941,6 +947,6 @@ def export(
         export_file(obj, outfile, fmt=metadata["data"].get("format", ""))
         logger.info("Actual file is:   %s", outfile)
 
-        export_metadata_file(metafile, metadata, savefmt=self.meta_format)
+        export_metadata_file(metafile, metadata)
         logger.info("Metadata file is: %s", metafile)
         return str(outfile)
diff --git a/src/fmu/dataio/preprocessed.py b/src/fmu/dataio/preprocessed.py
index b1151d085..d54e509d8 100644
--- a/src/fmu/dataio/preprocessed.py
+++ b/src/fmu/dataio/preprocessed.py
@@ -249,9 +249,7 @@ def export(self, obj: str | Path) -> str:
             else:
                 # store metafile to updated path
                 metafile = outfile.parent / f".{outfile.name}.yml"
-                export_metadata_file(
-                    file=metafile, metadata=meta_updated, savefmt="yaml"
-                )
+                export_metadata_file(file=metafile, metadata=meta_updated)
                 logger.info("Updated metadata file is: %s", metafile)
         else:
             warnings.warn(
diff --git a/tests/test_units/test_dataio.py b/tests/test_units/test_dataio.py
index 503255485..6d9ec9681 100644
--- a/tests/test_units/test_dataio.py
+++ b/tests/test_units/test_dataio.py
@@ -33,7 +33,7 @@ def test_generate_metadata_simple(globalconfig1):
 
     assert edata.config.model.name == "Test"
 
-    assert edata.meta_format == "yaml"
+    assert edata.meta_format is None
     assert edata.grid_fformat == "grdecl"
     assert edata.name == ""
 
@@ -973,24 +973,30 @@ def test_norwegian_letters_globalconfig(
     assert meta2["masterdata"]["smda"]["field"][0]["identifier"] == "DRÅGØN"
 
 
-def test_norwegian_letters_globalconfig_as_json(
-    globalvars_norwegian_letters,
-    regsurf,
-):
-    """Testing using norwegian letters in global config, with json output."""
-
-    path, cfg, _ = globalvars_norwegian_letters
-    os.chdir(path)
+def test_metadata_format_deprecated(globalconfig1, regsurf, tmp_path, monkeypatch):
+    """
+    Test that setting the class variable "metadata_format" gives a warning,
+    and that writing metadata on json format does not work
+    """
+    monkeypatch.chdir(tmp_path)
 
     ExportData.meta_format = "json"
-    edata = ExportData(config=cfg, name="TopBlåbær", content="depth")
+    with pytest.warns(UserWarning, match="meta_format"):
+        result = ExportData(
+            config=globalconfig1, name="TopBlåbær", content="depth"
+        ).export(regsurf)
 
-    result = pathlib.Path(edata.export(regsurf))
+    result = pathlib.Path(result)
     metafile = result.parent / ("." + str(result.stem) + ".gri.json")
-    with open(metafile, encoding="utf-8") as stream:
-        assert "DRÅGØN" in stream.read()
+    assert not metafile.exists()
+    assert not metafile.with_suffix(".yaml").exists()
+
+    # test that also value "yaml" will cause warning
+    ExportData.meta_format = "yaml"
+    with pytest.warns(UserWarning, match="meta_format"):
+        ExportData(config=globalconfig1, name="TopBlåbær", content="depth")
 
-    ExportData.meta_format = "yaml"  # reset
+    ExportData.meta_format = None  # reset
 
 
 def test_establish_runpath(tmp_path, globalconfig2):
diff --git a/tests/test_units/test_preprocessed.py b/tests/test_units/test_preprocessed.py
index 30d490972..555f5867c 100644
--- a/tests/test_units/test_preprocessed.py
+++ b/tests/test_units/test_preprocessed.py
@@ -130,7 +130,7 @@ def test_outdated_metadata(fmurun_prehook, rmsglobalconfig, regsurf, monkeypatch
     # modify existing metadata file to make it 'outdated'
     metadata = read_metadata(metafile)
     del metadata["data"]  # pretend data was not required before
-    utils.export_metadata_file(file=metafile, metadata=metadata, savefmt="yaml")
+    utils.export_metadata_file(file=metafile, metadata=metadata)
 
     # run the re-export of the preprocessed data inside an mocked FMU run
     set_ert_env_prehook(monkeypatch)
@@ -189,7 +189,7 @@ def test_preprocessed_surface_modified_post_export(
     # modify existing metadata file to make the md5sum inconsistent
     metadata = read_metadata(metafile)
     metadata["file"]["checksum_md5"] = "dummy_modified"
-    utils.export_metadata_file(file=metafile, metadata=metadata, savefmt="yaml")
+    utils.export_metadata_file(file=metafile, metadata=metadata)
 
     # run the re-export of the preprocessed data inside an mocked FMU run
     set_ert_env_prehook(monkeypatch)
diff --git a/tests/test_units/test_utils.py b/tests/test_units/test_utils.py
index 9e9c70b06..3fd3a2852 100644
--- a/tests/test_units/test_utils.py
+++ b/tests/test_units/test_utils.py
@@ -97,12 +97,7 @@ def test_non_metadata_export_metadata_file():
     with NamedTemporaryFile(buffering=0, suffix=".yaml") as tf, pytest.raises(
         RuntimeError
     ):
-        utils.export_metadata_file(Path(tf.name), {}, savefmt="json")
-
-    with NamedTemporaryFile(buffering=0, suffix=".yaml") as tf, pytest.raises(
-        RuntimeError
-    ):
-        utils.export_metadata_file(Path(tf.name), {}, savefmt="yaml")
+        utils.export_metadata_file(Path(tf.name), {})
 
 
 def test_export_file_raises():