diff --git a/packages/language-service/ivy/language_service.ts b/packages/language-service/ivy/language_service.ts
index 665607c9d0..04deedfda1 100644
--- a/packages/language-service/ivy/language_service.ts
+++ b/packages/language-service/ivy/language_service.ts
@@ -32,6 +32,18 @@ export class LanguageService {
   constructor(project: ts.server.Project, private readonly tsLS: ts.LanguageService) {
     this.parseConfigHost = new LSParseConfigHost(project.projectService.host);
     this.options = parseNgCompilerOptions(project, this.parseConfigHost);
+
+    // Projects loaded into the Language Service often include test files which are not part of the
+    // app's main compilation unit, and these test files often include inline NgModules that declare
+    // components from the app. These declarations conflict with the main declarations of such
+    // components in the app's NgModules. This conflict is not normally present during regular
+    // compilation because the app and the tests are part of separate compilation units.
+    //
+    // As a temporary mitigation of this problem, we instruct the compiler to ignore classes which
+    // are not exported. In many cases, this ensures the test NgModules are ignored by the compiler
+    // and only the real component declaration is used.
+    this.options.compileNonExportedClasses = false;
+
     this.strategy = createTypeCheckingProgramStrategy(project);
     this.adapter = new LanguageServiceAdapter(project);
     this.compilerFactory = new CompilerFactory(this.adapter, this.strategy, this.options);
diff --git a/packages/language-service/ivy/test/compiler_spec.ts b/packages/language-service/ivy/test/compiler_spec.ts
index 162502b880..98307bbce5 100644
--- a/packages/language-service/ivy/test/compiler_spec.ts
+++ b/packages/language-service/ivy/test/compiler_spec.ts
@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {absoluteFrom} from '@angular/compiler-cli/src/ngtsc/file_system';
+import {absoluteFrom, getSourceFileOrError} from '@angular/compiler-cli/src/ngtsc/file_system';
 import {initMockFileSystem} from '@angular/compiler-cli/src/ngtsc/file_system/testing';
 
 import {LanguageServiceTestEnvironment} from './env';
@@ -16,6 +16,63 @@ describe('language-service/compiler integration', () => {
     initMockFileSystem('Native');
   });
 
+  it('should not produce errors from inline test declarations mixing with those of the app', () => {
+    const appCmpFile = absoluteFrom('/test.cmp.ts');
+    const appModuleFile = absoluteFrom('/test.mod.ts');
+    const testFile = absoluteFrom('/test_spec.ts');
+
+    const env = LanguageServiceTestEnvironment.setup([
+      {
+        name: appCmpFile,
+        contents: `
+          import {Component} from '@angular/core';
+
+          @Component({
+            selector: 'app-cmp',
+            template: 'Some template',
+          })
+          export class AppCmp {}
+        `,
+        isRoot: true,
+      },
+      {
+        name: appModuleFile,
+        contents: `
+          import {NgModule} from '@angular/core';
+          import {AppCmp} from './test.cmp';
+
+          @NgModule({
+            declarations: [AppCmp],
+          })
+          export class AppModule {}
+        `,
+        isRoot: true,
+      },
+      {
+        name: testFile,
+        contents: `
+          import {NgModule} from '@angular/core';
+          import {AppCmp} from './test.cmp';
+
+          export function test(): void {
+            @NgModule({
+              declarations: [AppCmp],
+            })
+            class TestModule {}
+          }
+        `,
+        isRoot: true,
+      }
+    ]);
+
+    // Expect that this program is clean diagnostically.
+    const ngCompiler = env.ngLS.compilerFactory.getOrCreate();
+    const program = ngCompiler.getNextProgram();
+    expect(ngCompiler.getDiagnostics(getSourceFileOrError(program, appCmpFile))).toEqual([]);
+    expect(ngCompiler.getDiagnostics(getSourceFileOrError(program, appModuleFile))).toEqual([]);
+    expect(ngCompiler.getDiagnostics(getSourceFileOrError(program, testFile))).toEqual([]);
+  });
+
   it('should show type-checking errors from components with poisoned scopes', () => {
     // Normally, the Angular compiler suppresses errors from components that belong to NgModules
     // which themselves have errors (such scopes are considered "poisoned"), to avoid overwhelming