diff --git a/app/crud.py b/app/crud.py
index be9e5ba..9acd7f0 100644
--- a/app/crud.py
+++ b/app/crud.py
@@ -1,4 +1,5 @@
 from datetime import datetime
+import json
 import random
 import string
 from collections import defaultdict
@@ -531,6 +532,15 @@ def update_ballot(
     _check_election_is_started(db_election)
     _check_election_is_not_ended(db_election)
 
+    # Check if modification is allowed
+    modification_allowed = True
+    try:
+        desc_json = json.loads(str(db_election.description))
+        if isinstance(desc_json, dict) and "modificationAllowed" in desc_json:
+            modification_allowed = desc_json["modificationAllowed"]
+    except (json.JSONDecodeError, TypeError, ValueError):
+        pass
+
     if len(ballot.votes) != len(vote_ids):
         raise errors.ForbiddenError("Edit all votes at once.")
 
@@ -553,6 +563,13 @@ def update_ballot(
     if len(db_votes) != len(vote_ids):
         raise errors.NotFoundError("votes")
 
+    # If modification is NOT allowed, check if this is the first time voting
+    if not modification_allowed:
+        # A ballot is considered "voted" if at least one of its votes already has a grade_id
+        is_already_voted = any(v.grade_id is not None for v in db_votes)
+        if is_already_voted:
+            raise errors.VoteModificationForbiddenError()
+
     # Verify all votes belong to the same ballot
     ballot_ids = {int(v.ballot_id) for v in db_votes if v.ballot_id is not None}
 
diff --git a/app/errors.py b/app/errors.py
index a097802..cb3464f 100644
--- a/app/errors.py
+++ b/app/errors.py
@@ -99,3 +99,8 @@ class ElectionIsActiveError(CustomError):
     error_code = "ELECTION_IS_ACTIVE"
     message = "This election is already active and cannot be modified."
 
+class VoteModificationForbiddenError(CustomError):
+    status_code = 403
+    error_code = "VOTE_MODIFICATION_FORBIDDEN"
+    message = "Modification of votes is not allowed in this election."
+
diff --git a/app/tests/test_api.py b/app/tests/test_api.py
index 3c68172..2b579bb 100644
--- a/app/tests/test_api.py
+++ b/app/tests/test_api.py
@@ -77,6 +77,7 @@ class RandomElection(t.TypedDict):
     date_end: t.Optional[str]
     date_start: t.Optional[str]
     auth_for_result: bool
+    description: t.Optional[str]
 
 
 def _random_election(num_candidates: int, num_grades: int) -> RandomElection:
@@ -98,6 +99,7 @@ def _random_election(num_candidates: int, num_grades: int) -> RandomElection:
         "num_voters": 0,
         "date_end": None,
         "auth_for_result": False,
+        "description": "",
     }
 
 
@@ -943,3 +945,49 @@ def test_progress():
     assert progress_rep.status_code == 200, progress_data
     assert progress_data["num_voters"] == 10
     assert progress_data["num_voters_voted"] == 1
+
+def test_vote_modification_restriction():
+    """
+    Tests that a vote cannot be modified if the election's modificationAllowed is False.
+    """
+    import json
+    # Create a restricted election with modificationAllowed: False in description
+    body = _random_election(5, 3)
+    body["restricted"] = True
+    body["num_voters"] = 1
+    # Store modificationAllowed in the JSON description
+    body["description"] = json.dumps({
+        "description": "This is a test election",
+        "modificationAllowed": False
+    })
+
+    response = client.post("/elections", json=body)
+    assert response.status_code == 200, response.text
+    election_data = response.json()
+    ballot_token = election_data["invites"][0]
+
+    # Cast initial vote
+    grade_id = election_data["grades"][0]["id"]
+    votes = [
+        {"candidate_id": c["id"], "grade_id": grade_id}
+        for c in election_data["candidates"]
+    ]
+    response = client.put(
+        "/ballots",
+        json={"votes": votes},
+        headers={"Authorization": f"Bearer {ballot_token}"}
+    )
+    assert response.status_code == 200, "Initial vote failed"
+
+    # Try to modify the vote
+    new_grade_id = election_data["grades"][1]["id"]
+    new_votes = [
+        {"candidate_id": c["id"], "grade_id": new_grade_id}
+        for c in election_data["candidates"]
+    ]
+    response = client.put(
+        "/ballots",
+        json={"votes": new_votes},
+        headers={"Authorization": f"Bearer {ballot_token}"}
+    )
+    check_error_response(response, 403, "VOTE_MODIFICATION_FORBIDDEN")