diff --git a/apps/backoffice/src/app/features/exam/pages/ExamDetailPage.tsx b/apps/backoffice/src/app/features/exam/pages/ExamDetailPage.tsx
index 4abe06117..ff8759458 100644
--- a/apps/backoffice/src/app/features/exam/pages/ExamDetailPage.tsx
+++ b/apps/backoffice/src/app/features/exam/pages/ExamDetailPage.tsx
@@ -22,6 +22,7 @@ import {
   TextInput,
   ThemeIcon,
   Box,
+  Tooltip,
   rem,
 } from "@mantine/core";
 import { useDisclosure } from "@mantine/hooks";
@@ -48,6 +49,7 @@ import {
   useUpdateExamMutation,
   useAssignQuestionsMutation,
   useSelectRandomQuestionsMutation,
+  useGetExamRegistrationsQuery,
 } from '../api/exam-api';
 import { useGetQuestionsQuery } from '../../question/api/question-api';
 import { useGetCertificationsQuery } from '../../certification/api/certification-api';
@@ -114,6 +116,12 @@ export function ExamDetailPage() {
   const [selectRandom, { isLoading: isDrawing }] = useSelectRandomQuestionsMutation();
 
   const { data: exam, isLoading, isError } = useGetExamQuery(id ?? '', { skip: !id });
+  // The backend locks the paper the moment the first candidate registers
+  // (ExamService.assertPaperEditable) — every candidate must sit the same
+  // paper. Same query ExamCandidatesPanel already runs, so RTK Query serves
+  // it from cache rather than issuing a second request.
+  const { data: registrations } = useGetExamRegistrationsQuery(id ?? '', { skip: !id });
+  const paperLocked = (registrations?.length ?? 0) > 0;
   const { data: qRes } = useGetQuestionsQuery();
   const { data: certRes } = useGetCertificationsQuery();
   const allQuestions = qRes?.items ?? [];
@@ -183,7 +191,9 @@ export function ExamDetailPage() {
     } catch (error) {
       const key = extractErrorMessage(error, t('exam.randomError'));
       notify.error(
-        key.startsWith('insufficient_approved_questions')
+        key === 'paper_locked_after_registration'
+          ? t('exam.paperLockedHint', { count: registrations?.length ?? 0 })
+          : key.startsWith('insufficient_approved_questions')
           ? `${t('exam.notEnoughApproved')} (${key.split(':')[1] ?? ''})`
           : key.startsWith('paper_cannot_reach_cutting_point')
             ? t('exam.cannotReachCuttingPoint', {
@@ -204,7 +214,9 @@ export function ExamDetailPage() {
     } catch (error) {
       const key = extractErrorMessage(error, 'Failed to assign questions');
       notify.error(
-        key.startsWith('question_not_approved')
+        key === 'paper_locked_after_registration'
+          ? t('exam.paperLockedHint', { count: registrations?.length ?? 0 })
+          : key.startsWith('question_not_approved')
           ? t('question.qc.onlyApprovedUsable')
           : key.startsWith('paper_cannot_reach_cutting_point')
             ? t('exam.cannotReachCuttingPoint', {
@@ -456,19 +468,45 @@ export function ExamDetailPage() {
             {t("exam.detail.questionsSection", { pts: totalPoints })}
           </Title>
           <RequirePermission anyOf={[LICENSE_PERMISSIONS.MANAGE_EXAMS]} hideOnly>
-            <Button
-              variant="light"
-              size="xs"
-              leftSection={<IconPlus size={14} />}
-              onClick={openAssignModal}
-            >
-              {t("exam.manageQuestions")}
-            </Button>
+            <Group gap="xs">
+              {paperLocked && (
+                <Badge size="sm" variant="light" color="gray">
+                  {t("exam.paperLocked")}
+                </Badge>
+              )}
+              <Tooltip
+                label={t("exam.paperLockedHint", {
+                  count: registrations?.length ?? 0,
+                })}
+                disabled={!paperLocked}
+                multiline
+                w={280}
+              >
+                {/* Wrapped: a disabled Mantine Button fires no pointer events,
+                    so the tooltip needs an enabled element to hang off. */}
+                <Box>
+                  <Button
+                    variant="light"
+                    size="xs"
+                    leftSection={<IconPlus size={14} />}
+                    onClick={openAssignModal}
+                    disabled={paperLocked}
+                  >
+                    {t("exam.manageQuestions")}
+                  </Button>
+                </Box>
+              </Tooltip>
+            </Group>
           </RequirePermission>
         </Group>
         {(exam.questions ?? []).length === 0 ? (
-          <Alert color="gray" icon={<IconInfoCircle size={16} />}>
-            {t("exam.noQuestionsAssigned")}
+          <Alert
+            color={paperLocked ? "red" : "gray"}
+            icon={<IconInfoCircle size={16} />}
+          >
+            {paperLocked
+              ? t("exam.paperLockedHint", { count: registrations?.length ?? 0 })
+              : t("exam.noQuestionsAssigned")}
           </Alert>
         ) : (
           <Stack gap="md">
diff --git a/apps/backoffice/src/app/i18n/locales/am.ts b/apps/backoffice/src/app/i18n/locales/am.ts
index 292a374ae..c8e51193d 100644
--- a/apps/backoffice/src/app/i18n/locales/am.ts
+++ b/apps/backoffice/src/app/i18n/locales/am.ts
@@ -387,6 +387,9 @@ export const am: Translations = {
     notEnoughApproved: "ለዚህ ትምህርት በቂ የጸደቁ ጥያቄዎች የሉም።",
     cannotReachCuttingPoint:
       "ይህ ወረቀት የማለፊያ ነጥቡን ሊደርስ አይችልም (ከፍተኛ {{max}}፣ የማለፊያ ነጥብ {{cuttingPoint}})። ተጨማሪ ወይም ከፍ ያለ ነጥብ ያላቸው ጥያቄዎችን ጨምር፣ ወይም የማለፊያ ነጥቡን ቀንስ።",
+    paperLocked: "ወረቀቱ ተቆልፏል",
+    paperLockedHint:
+      "ለዚህ ፈተና {{count}} ተፈታኝ(ዎች) ተመዝግበዋል። ሁሉም ተፈታኞች አንድ ዓይነት ወረቀት መፈተን ስላለባቸው ጥያቄዎቹ ከዚህ በኋላ አይቀየሩም።",
   },
 
   country: {
diff --git a/apps/backoffice/src/app/i18n/locales/en.ts b/apps/backoffice/src/app/i18n/locales/en.ts
index 8c0ff66ff..7152067ce 100644
--- a/apps/backoffice/src/app/i18n/locales/en.ts
+++ b/apps/backoffice/src/app/i18n/locales/en.ts
@@ -386,6 +386,9 @@ export const en = {
       'Not enough approved questions in the bank for this subject.',
     cannotReachCuttingPoint:
       'This paper cannot reach the passing mark (max {{max}}, pass mark {{cuttingPoint}}). Add more/higher-point questions, or lower the cutting point.',
+    paperLocked: 'Paper locked',
+    paperLockedHint:
+      '{{count}} candidate(s) have registered for this session. Every candidate must sit the same paper, so questions can no longer be changed.',
   },
 
   country: {