diff --git a/packages/core/e2e/graphql/generated-e2e-admin-types.ts b/packages/core/e2e/graphql/generated-e2e-admin-types.ts
index 95ef3dffdc..dc39bccf97 100644
--- a/packages/core/e2e/graphql/generated-e2e-admin-types.ts
+++ b/packages/core/e2e/graphql/generated-e2e-admin-types.ts
@@ -4475,7 +4475,7 @@ export type GetCustomerQuery = { __typename?: 'Query' } & {
 export type AttemptLoginMutationVariables = {
     username: Scalars['String'];
     password: Scalars['String'];
-    rememberMe: Scalars['Boolean'];
+    rememberMe?: Maybe<Scalars['Boolean']>;
 };
 
 export type AttemptLoginMutation = { __typename?: 'Mutation' } & {
diff --git a/packages/core/e2e/graphql/shared-definitions.ts b/packages/core/e2e/graphql/shared-definitions.ts
index e4c4464a82..f4a80a1ee4 100644
--- a/packages/core/e2e/graphql/shared-definitions.ts
+++ b/packages/core/e2e/graphql/shared-definitions.ts
@@ -194,7 +194,7 @@ export const GET_CUSTOMER = gql`
 `;
 
 export const ATTEMPT_LOGIN = gql`
-    mutation AttemptLogin($username: String!, $password: String!, $rememberMe: Boolean!) {
+    mutation AttemptLogin($username: String!, $password: String!, $rememberMe: Boolean) {
         login(username: $username, password: $password, rememberMe: $rememberMe) {
             user {
                 ...CurrentUser
diff --git a/packages/core/e2e/shop-order.e2e-spec.ts b/packages/core/e2e/shop-order.e2e-spec.ts
index 71c1f77590..dc411e3ce6 100644
--- a/packages/core/e2e/shop-order.e2e-spec.ts
+++ b/packages/core/e2e/shop-order.e2e-spec.ts
@@ -12,6 +12,7 @@ import {
     testSuccessfulPaymentMethod,
 } from './fixtures/test-payment-methods';
 import {
+    AttemptLogin,
     CreateAddressInput,
     GetCountryList,
     GetCustomer,
@@ -36,6 +37,7 @@ import {
     TransitionToState,
 } from './graphql/generated-e2e-shop-types';
 import {
+    ATTEMPT_LOGIN,
     GET_COUNTRY_LIST,
     GET_CUSTOMER,
     GET_CUSTOMER_LIST,
@@ -80,7 +82,7 @@ describe('Shop orders', () => {
         await server.init({
             initialData,
             productsCsvPath: path.join(__dirname, 'fixtures/e2e-products-full.csv'),
-            customerCount: 2,
+            customerCount: 3,
         });
         await adminClient.asSuperAdmin();
     }, TEST_SETUP_TIMEOUT_MS);
@@ -874,4 +876,93 @@ describe('Shop orders', () => {
             });
         });
     });
+
+    describe('order merging', () => {
+        let customers: GetCustomerList.Items[];
+
+        beforeAll(async () => {
+            const result = await adminClient.query<GetCustomerList.Query>(GET_CUSTOMER_LIST);
+            customers = result.customers.items;
+        });
+
+        it('merges guest order with no existing order', async () => {
+            await shopClient.asAnonymousUser();
+            const { addItemToOrder } = await shopClient.query<
+                AddItemToOrder.Mutation,
+                AddItemToOrder.Variables
+            >(ADD_ITEM_TO_ORDER, {
+                productVariantId: 'T_1',
+                quantity: 1,
+            });
+
+            expect(addItemToOrder!.lines.length).toBe(1);
+            expect(addItemToOrder!.lines[0].productVariant.id).toBe('T_1');
+
+            await shopClient.query<AttemptLogin.Mutation, AttemptLogin.Variables>(ATTEMPT_LOGIN, {
+                username: customers[1].emailAddress,
+                password: 'test',
+            });
+            const { activeOrder } = await shopClient.query<GetActiveOrder.Query>(GET_ACTIVE_ORDER);
+
+            expect(activeOrder!.lines.length).toBe(1);
+            expect(activeOrder!.lines[0].productVariant.id).toBe('T_1');
+        });
+
+        it('merges guest order with existing order', async () => {
+            await shopClient.asAnonymousUser();
+            const { addItemToOrder } = await shopClient.query<
+                AddItemToOrder.Mutation,
+                AddItemToOrder.Variables
+            >(ADD_ITEM_TO_ORDER, {
+                productVariantId: 'T_2',
+                quantity: 1,
+            });
+
+            expect(addItemToOrder!.lines.length).toBe(1);
+            expect(addItemToOrder!.lines[0].productVariant.id).toBe('T_2');
+
+            await shopClient.query<AttemptLogin.Mutation, AttemptLogin.Variables>(ATTEMPT_LOGIN, {
+                username: customers[1].emailAddress,
+                password: 'test',
+            });
+            const { activeOrder } = await shopClient.query<GetActiveOrder.Query>(GET_ACTIVE_ORDER);
+
+            expect(activeOrder!.lines.length).toBe(2);
+            expect(activeOrder!.lines[0].productVariant.id).toBe('T_1');
+            expect(activeOrder!.lines[1].productVariant.id).toBe('T_2');
+        });
+
+        /**
+         * See https://github.com/vendure-ecommerce/vendure/issues/263
+         */
+        it('does not merge when logging in to a different account (issue #263)', async () => {
+            await shopClient.query<AttemptLogin.Mutation, AttemptLogin.Variables>(ATTEMPT_LOGIN, {
+                username: customers[2].emailAddress,
+                password: 'test',
+            });
+            const { activeOrder } = await shopClient.query<GetActiveOrder.Query>(GET_ACTIVE_ORDER);
+
+            expect(activeOrder).toBeNull();
+        });
+
+        it('does not merge when logging back to other account (issue #263)', async () => {
+            const { addItemToOrder } = await shopClient.query<
+                AddItemToOrder.Mutation,
+                AddItemToOrder.Variables
+            >(ADD_ITEM_TO_ORDER, {
+                productVariantId: 'T_3',
+                quantity: 1,
+            });
+
+            await shopClient.query<AttemptLogin.Mutation, AttemptLogin.Variables>(ATTEMPT_LOGIN, {
+                username: customers[1].emailAddress,
+                password: 'test',
+            });
+            const { activeOrder } = await shopClient.query<GetActiveOrder.Query>(GET_ACTIVE_ORDER);
+
+            expect(activeOrder!.lines.length).toBe(2);
+            expect(activeOrder!.lines[0].productVariant.id).toBe('T_1');
+            expect(activeOrder!.lines[1].productVariant.id).toBe('T_2');
+        });
+    });
 });
diff --git a/packages/core/src/service/services/order.service.ts b/packages/core/src/service/services/order.service.ts
index 1f38523132..bfef3f62fe 100644
--- a/packages/core/src/service/services/order.service.ts
+++ b/packages/core/src/service/services/order.service.ts
@@ -717,6 +717,11 @@ export class OrderService {
         guestOrder?: Order,
         existingOrder?: Order,
     ): Promise<Order | undefined> {
+        if (guestOrder && guestOrder.customer) {
+            // In this case the "guest order" is actually an order of an existing Customer,
+            // so we do not want to merge at all. See https://github.com/vendure-ecommerce/vendure/issues/263
+            return existingOrder;
+        }
         const mergeResult = await this.orderMerger.merge(guestOrder, existingOrder);
         const { orderToDelete, linesToInsert } = mergeResult;
         let { order } = mergeResult;