diff --git a/newsfragments/3403.deprecated.rst b/newsfragments/3403.deprecated.rst
new file mode 100644
index 000000000..4efb61bc7
--- /dev/null
+++ b/newsfragments/3403.deprecated.rst
@@ -0,0 +1,6 @@
+Trying to use absolute deadlines on a `trio.CancelScope` constructed
+with a relative deadline now raises, after being deprecated since Trio
+0.27.0.
+
+You can still set absolute deadlines or relative deadlines to change
+the kind of `trio.CancelScope` you have.
diff --git a/src/trio/_core/_run.py b/src/trio/_core/_run.py
index 2d649d291..ffd839406 100644
--- a/src/trio/_core/_run.py
+++ b/src/trio/_core/_run.py
@@ -808,29 +808,17 @@ def deadline(self) -> float:
         """
         if self._relative_deadline != inf:
             assert self._deadline == inf
-            warnings.warn(
-                DeprecationWarning(
-                    "unentered relative cancel scope does not have an absolute deadline. Use `.relative_deadline`",
-                ),
-                stacklevel=2,
+            raise RuntimeError(
+                "Unentered relative cancel scope does not have an absolute deadline."
             )
-            return current_time() + self._relative_deadline
         return self._deadline
 
     @deadline.setter
     def deadline(self, new_deadline: float) -> None:
         if isnan(new_deadline):
             raise ValueError("deadline must not be NaN")
-        if self._relative_deadline != inf:
-            assert self._deadline == inf
-            warnings.warn(
-                DeprecationWarning(
-                    "unentered relative cancel scope does not have an absolute deadline. Transforming into an absolute cancel scope. First set `.relative_deadline = math.inf` if you do want an absolute cancel scope.",
-                ),
-                stacklevel=2,
-            )
-            self._relative_deadline = inf
         with self._might_change_registered_deadline():
+            self._relative_deadline = inf
             self._deadline = float(new_deadline)
 
     @property
@@ -852,7 +840,7 @@ def relative_deadline(self) -> float:
         elif self._deadline != inf:
             assert self._relative_deadline == inf
             raise RuntimeError(
-                "unentered non-relative cancel scope does not have a relative deadline",
+                "Unentered non-relative cancel scope does not have a relative deadline",
             )
         return self._relative_deadline
 
@@ -865,13 +853,10 @@ def relative_deadline(self, new_relative_deadline: float) -> None:
         if self._has_been_entered:
             with self._might_change_registered_deadline():
                 self._deadline = current_time() + float(new_relative_deadline)
-        elif self._deadline != inf:
-            assert self._relative_deadline == inf
-            raise RuntimeError(
-                "unentered non-relative cancel scope does not have a relative deadline",
-            )
         else:
-            self._relative_deadline = new_relative_deadline
+            with self._might_change_registered_deadline():
+                self._deadline = inf
+                self._relative_deadline = new_relative_deadline
 
     @property
     def is_relative(self) -> bool | None:
diff --git a/src/trio/_tests/test_timeouts.py b/src/trio/_tests/test_timeouts.py
index ff62a0708..3eb545002 100644
--- a/src/trio/_tests/test_timeouts.py
+++ b/src/trio/_tests/test_timeouts.py
@@ -243,34 +243,31 @@ async def test_timeout_deadline_on_entry(mock_clock: _core.MockClock) -> None:
         assert rcs is cs
 
 
-async def test_invalid_access_unentered(mock_clock: _core.MockClock) -> None:
+def test_invalid_access_unentered() -> None:
     cs = move_on_after(5)
-    mock_clock.jump(3)
-    start = _core.current_time()
 
-    match_str = "^unentered relative cancel scope does not have an absolute deadline"
-    with pytest.warns(DeprecationWarning, match=match_str):
-        assert cs.deadline == start + 5
-    mock_clock.jump(1)
-    # this is hella sketchy, but they *have* been warned
-    with pytest.warns(DeprecationWarning, match=match_str):
-        assert cs.deadline == start + 6
-
-    with pytest.warns(DeprecationWarning, match=match_str):
-        cs.deadline = 7
-    # now transformed into absolute
+    match_str = "^Unentered relative cancel scope does not have an absolute deadline"
+    with pytest.raises(RuntimeError, match=match_str):
+        assert cs.deadline
+    with pytest.raises(RuntimeError, match=match_str):
+        cs.deadline += 5
+
+    # switch from relative to absolute
+    cs.deadline = 7
     assert cs.deadline == 7
     assert not cs.is_relative
 
-    cs = move_on_at(5)
-
     match_str = (
-        "^unentered non-relative cancel scope does not have a relative deadline$"
+        "^Unentered non-relative cancel scope does not have a relative deadline$"
     )
     with pytest.raises(RuntimeError, match=match_str):
         assert cs.relative_deadline
     with pytest.raises(RuntimeError, match=match_str):
-        cs.relative_deadline = 7
+        cs.relative_deadline += 5
+
+    cs.relative_deadline = 9
+    assert cs.relative_deadline == 9
+    assert cs.is_relative
 
 
 @pytest.mark.xfail(reason="not implemented")