diff --git a/.github/workflows/python-ci.yml b/.github/workflows/python-ci.yml
new file mode 100644
index 0000000..e07c82c
--- /dev/null
+++ b/.github/workflows/python-ci.yml
@@ -0,0 +1,31 @@
+name: python-ci-main
+
+on:
+  pull_request:
+    paths:
+      - '/versions/v1/units.json'
+    branches:
+      - main
+
+jobs:
+  compare-json:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout main
+      uses: actions/checkout@v3
+      with:
+        ref: main
+        path: main
+
+    - name: Checkout PR
+      uses: actions/checkout@v3
+      with:
+        path: pr
+
+    - name: Set up Python
+      uses: actions/setup-python@v4
+      with:
+        python-version: '3.10'
+
+    - name: Compare JSON
+      run: python ./src/compare_external_ids.py
diff --git a/README.md b/README.md
index 8652107..5489e82 100644
--- a/README.md
+++ b/README.md
@@ -68,6 +68,7 @@ To ensure the integrity of the catalog, the following tests are conducted:
 5. **Consistent References**: All quantity references in `unitSystems.json` must exist in `units.json`.
 6. **Unique aliases**: All pairs of (`alias` and `quantity`) must be unique, for all **aliases** in `aliasNames`.
 7. **ExternalId Format**: All unit `externalIds` must follow the pattern `{quantity}:{unit}`, where both `quantity` and `unit` are in **snake_case**.
+8. **Avoid breaking changes**: There can be no removals of unit `externalIds` in `units.json`. Only additions are supported.
 
 ## Attribution
 Some of the units are sourced from QUDT.org, which is licensed under the [Creative Commons Attribution 4.0 International License](https://creativecommons.org/licenses/by/4.0/).
diff --git a/src/compare_external_ids.py b/src/compare_external_ids.py
new file mode 100644
index 0000000..95a43ce
--- /dev/null
+++ b/src/compare_external_ids.py
@@ -0,0 +1,20 @@
+import json
+
+
+def get_external_ids(file_path: str) -> set:
+    with open(file_path, "r") as f:
+        data = json.load(f)
+    return set(item["externalId"] for item in data)
+
+
+main_external_ids: set = get_external_ids("main/versions/v1/units.json")
+pr_external_ids: set = get_external_ids("pr/versions/v1/units.json")
+
+# 8. Avoid breaking changes: There can be no removals of unit `externalIds` in `units.json`.
+# Only additions are supported.
+removed_external_ids: set = main_external_ids - pr_external_ids
+
+if removed_external_ids:
+    raise Exception(
+        f'externalId(s) [{", ".join(removed_external_ids)}] have been removed from the unit catalogue.'
+    )
diff --git a/versions/v1/units.json b/versions/v1/units.json
index ba0edf8..e553a77 100644
--- a/versions/v1/units.json
+++ b/versions/v1/units.json
@@ -47,55 +47,6 @@
     "source": "qudt.org",
     "sourceReference": "http://qudt.org/vocab/unit/FT-PER-SEC2"
   },
-  {
-    "externalId": "acceleration:in-per-sec2",
-    "name": "IN-PER-SEC2",
-    "quantity": "Acceleration",
-    "longName": "Inch per Square second",
-    "aliasNames": [
-      "in per square second",
-      "Inch / SEC2",
-      "IN / square second",
-      "IN PER SEC2",
-      "inch / SEC2",
-      "Inch per s²",
-      "in / Square Second",
-      "inch / s²",
-      "in / square second",
-      "Inch per Square Second",
-      "Inch / Square Second",
-      "Inch / square second",
-      "Inch / s²",
-      "inch / Square Second",
-      "in / SEC2",
-      "in / s²",
-      "inch / square second",
-      "inch per square second",
-      "Inch per Square second",
-      "Inch per SEC2",
-      "in per Square Second",
-      "Inch per square second",
-      "inch per Square Second",
-      "IN per Square Second",
-      "inch per s²",
-      "in per s²",
-      "IN / s²",
-      "IN per s²",
-      "inch per SEC2",
-      "IN / Square Second",
-      "in per SEC2",
-      "IN per square second",
-      "IN per SEC2",
-      "IN / SEC2"
-    ],
-    "symbol": "in/s²",
-    "conversion": {
-      "multiplier": 0.0254,
-      "offset": 0.0
-    },
-    "source": "qudt.org",
-    "sourceReference": "http://qudt.org/vocab/unit/IN-PER-SEC2"
-  },
   {
     "externalId": "acceleration:m-per-sec2",
     "name": "M-PER-SEC2",