diff --git a/sumo/io/castep.py b/sumo/io/castep.py
index b297e7d5..c8416349 100644
--- a/sumo/io/castep.py
+++ b/sumo/io/castep.py
@@ -100,9 +100,11 @@ def structure(self):
             return Structure(lattice, elements, coords, coords_are_cartesian=False)
         elif "positions_abs" in self.blocks:
             if "lattice_abc" in self.blocks:
-                logger.warning(
-                    "Positions are given in absolute coordinates and lattice "
-                    "in angles/lengths format. Structure may not be consistent."
+                raise ValueError(
+                    "Positions are given in Cartesian coordinates and lattice "
+                    "in angles/lengths format. This structure cannot be "
+                    "interpreted reliably. Consider using c2x to convert to "
+                    "fractional coordinates and/or 3x3 lattice matrix."
                 )
 
             positions_abs = self.blocks["positions_abs"].values
diff --git a/tests/data/NiO/NiO_abc.cell b/tests/data/NiO/NiO_abc.cell
index 46a84eee..e63cb4bc 100644
--- a/tests/data/NiO/NiO_abc.cell
+++ b/tests/data/NiO/NiO_abc.cell
@@ -3,12 +3,12 @@
 106.80215981  73.19772637 119.99807072
 %ENDBLOCK LATTICE_ABC
 
-%BLOCK POSITIONS_ABS
+%BLOCK POSITIONS_FRAC
 Ni  0.000000  0.000000  0.000000 SPIN=1
-Ni  1.491603  0.861143  2.432002 SPIN=-1
-O  0.000069  1.722313  1.215967  SPIN=0
-O  2.983138 -0.000026  3.648036  SPIN=0
-%ENDBLOCK POSITIONS_ABS
+Ni  0.500000  0.500000  0.500000 SPIN=-1
+O  0.250001  0.749999  0.249993  SPIN=0
+O  0.749999  0.250001  0.750007  SPIN=0
+%ENDBLOCK POSITIONS_FRAC
 
 %BLOCK SPECIES_POT
 Ni C19
diff --git a/tests/data/NiO/NiO_abc_units.cell b/tests/data/NiO/NiO_abc_units.cell
index d89d422d..bfa79556 100644
--- a/tests/data/NiO/NiO_abc_units.cell
+++ b/tests/data/NiO/NiO_abc_units.cell
@@ -4,13 +4,12 @@ Ang
 106.80215981  73.19772637 119.99807072
 %ENDBLOCK LATTICE_ABC
 
-%BLOCK POSITIONS_ABS
-Ang
+%BLOCK POSITIONS_FRAC
 Ni  0.000000  0.000000  0.000000 SPIN=1
-Ni  1.491603  0.861143  2.432002 SPIN=-1
-O  0.000069  1.722313  1.215967  SPIN=0
-O  2.983138 -0.000026  3.648036  SPIN=0
-%ENDBLOCK POSITIONS_ABS
+Ni  0.500000  0.500000  0.500000 SPIN=-1
+O  0.250001  0.749999  0.249993  SPIN=0
+O  0.749999  0.250001  0.750007  SPIN=0
+%ENDBLOCK POSITIONS_FRAC
 
 %BLOCK SPECIES_POT
 Ni C19
@@ -19,4 +18,3 @@ O  C19
 
 KPOINTS_MP_GRID 7 7 7
 SPECTRAL_KPOINTS_MP_GRID 2 2 2
-
diff --git a/tests/tests_io/test_castep.py b/tests/tests_io/test_castep.py
index 70404b4c..9f3e78e3 100644
--- a/tests/tests_io/test_castep.py
+++ b/tests/tests_io/test_castep.py
@@ -38,12 +38,19 @@ def setUp(self):
         self.zns_singlepoint_cell = os.path.join(
             ilr_files("tests"), "data", "ZnS", "zns-sp.cell"
         )
+        self.nio_cart_cell = os.path.join(
+            ilr_files("tests"), "data", "NiO", "NiO.cell"
+        )
         self.nio_abc_cell = os.path.join(
             ilr_files("tests"), "data", "NiO", "NiO_abc.cell"
         )
         self.nio_abc_units_cell = os.path.join(
             ilr_files("tests"), "data", "NiO", "NiO_abc_units.cell"
         )
+        self.nio_abc_cart_cell = os.path.join(
+            ilr_files("tests"), "data", "NiO", "NiO_abc_cart.cell"
+        )
+
 
     def test_castep_cell_null_init(self):
         null_cell = CastepCell()
@@ -82,20 +89,33 @@ def test_castep_cell_from_singlepoint_file(self):
     def test_castep_cell_abc(self):
         """Test .cell file using lattice_abc to define unit cell"""
         for filename in self.nio_abc_cell, self.nio_abc_units_cell:
-            with self.assertLogs(logger) as log:
-                cc = CastepCell.from_file(filename)
-                structure = cc.structure
-                self.assertIn("Structure may not be consistent.", log.output[-1])
+            cc = CastepCell.from_file(filename)
+            structure = cc.structure
 
             assert_array_almost_equal(
-                structure.lattice.matrix,
-                [
-                    [2.855724, 0.0, 0.862317],
-                    [-1.297582, 2.54391, -0.862313],
-                    [0.0, 0.0, 5.159941],
-                ],
+                structure.lattice.abc,
+                [2.983077, 2.98308198, 5.15994087]
+            )
+            assert_array_almost_equal(
+                structure.lattice.angles,
+                [106.80215981, 73.19772637, 119.99807072]
             )
 
+    def test_castep_cell_consistent_frac_coords(self):
+        """Test fractional positions are consistent between .cell formats"""
+        cartesian_cell = CastepCell.from_file(self.nio_cart_cell)
+        frac_abc_cell = CastepCell.from_file(self.nio_abc_cell)
+
+        assert_array_almost_equal(
+            cartesian_cell.structure.frac_coords,
+            frac_abc_cell.structure.frac_coords
+        )
+
+    def test_castep_abc_cart_raises(self):
+        """Test that error is raised for incompatible vectors and positions"""
+        with self.assertRaisesRegex(ValueError, "Cartesian"):
+            CastepCell.from_file(self.nio_abc_cart_cell).structure
+
     def test_castep_cell_from_structure(self):
         cell = CastepCell.from_structure(self.si_structure)
         self.assertEqual(