diff --git a/README.md b/README.md
index a9508e70a..1f1a662f4 100644
--- a/README.md
+++ b/README.md
@@ -234,6 +234,10 @@ same major line. Should you need to upgrade to a new major, use an explicit
   not to lookup on the remote registry for the latest version of the selected
   package manager.
 
+- `COREPACK_ENABLE_URL_VERSION_FOR_KNOWN_PM` can be set to `1` to allow use of
+  custom URLs to load a package manager known by Corepack (`yarn`, `npm`, and
+  `pnpm`).
+
 - `COREPACK_ENABLE_NETWORK` can be set to `0` to prevent Corepack from accessing
   the network (in which case you'll be responsible for hydrating the package
   manager versions that will be required for the projects you'll run, using
diff --git a/sources/main.ts b/sources/main.ts
index 9b880ee05..a15316e20 100644
--- a/sources/main.ts
+++ b/sources/main.ts
@@ -35,9 +35,9 @@ function getPackageManagerRequestFromCli(parameter: string | undefined, context:
     return null;
 
   const [, binaryName, binaryVersion] = match;
-  const packageManager = context.engine.getPackageManagerFor(binaryName);
-  if (!packageManager)
-    return null;
+  const packageManager = context.engine.getPackageManagerFor(binaryName)!;
+
+  if (packageManager == null && binaryVersion == null) return null;
 
   return {
     packageManager,
@@ -47,28 +47,34 @@ function getPackageManagerRequestFromCli(parameter: string | undefined, context:
 }
 
 async function executePackageManagerRequest({packageManager, binaryName, binaryVersion}: PackageManagerRequest, args: Array<string>, context: Context) {
-  const defaultVersion = await context.engine.getDefaultVersion(packageManager);
-  const definition = context.engine.config.definitions[packageManager]!;
-
-  // If all leading segments match one of the patterns defined in the `transparent`
-  // key, we tolerate calling this binary even if the local project isn't explicitly
-  // configured for it, and we use the special default version if requested.
+  let fallbackLocator: Locator = {
+    name: binaryName,
+    reference: undefined as any,
+  };
   let isTransparentCommand = false;
-  for (const transparentPath of definition.transparent.commands) {
-    if (transparentPath[0] === binaryName && transparentPath.slice(1).every((segment, index) => segment === args[index])) {
-      isTransparentCommand = true;
-      break;
+  if (packageManager != null) {
+    const defaultVersion = await context.engine.getDefaultVersion(packageManager);
+    const definition = context.engine.config.definitions[packageManager]!;
+
+    // If all leading segments match one of the patterns defined in the `transparent`
+    // key, we tolerate calling this binary even if the local project isn't explicitly
+    // configured for it, and we use the special default version if requested.
+    for (const transparentPath of definition.transparent.commands) {
+      if (transparentPath[0] === binaryName && transparentPath.slice(1).every((segment, index) => segment === args[index])) {
+        isTransparentCommand = true;
+        break;
+      }
     }
-  }
 
-  const fallbackReference = isTransparentCommand
-    ? definition.transparent.default ?? defaultVersion
-    : defaultVersion;
+    const fallbackReference = isTransparentCommand
+      ? definition.transparent.default ?? defaultVersion
+      : defaultVersion;
 
-  const fallbackLocator: Locator = {
-    name: packageManager,
-    reference: fallbackReference,
-  };
+    fallbackLocator = {
+      name: packageManager,
+      reference: fallbackReference,
+    };
+  }
 
   let descriptor: Descriptor;
   try {
diff --git a/sources/specUtils.ts b/sources/specUtils.ts
index fcafdf0c2..a1ee17756 100644
--- a/sources/specUtils.ts
+++ b/sources/specUtils.ts
@@ -38,6 +38,8 @@ export function parseSpec(raw: unknown, source: string, {enforceExactVersion = t
     if (!isSupportedPackageManager(name)) {
       throw new UsageError(`Unsupported package manager specification (${raw})`);
     }
+  } else if (isSupportedPackageManager(name) && process.env.COREPACK_ENABLE_URL_VERSION_FOR_KNOWN_PM !== `1`) {
+    throw new UsageError(`Illegal use of URL for known package manager. Instead, select a specific version, or set COREPACK_ENABLE_URL_VERSION_FOR_KNOWN_PM=1 in your environment (${raw})`);
   }
 
 
diff --git a/tests/_runCli.ts b/tests/_runCli.ts
index e028af751..f863c10b0 100644
--- a/tests/_runCli.ts
+++ b/tests/_runCli.ts
@@ -1,7 +1,7 @@
 import {PortablePath, npath} from '@yarnpkg/fslib';
 import {spawn}               from 'child_process';
 
-export async function runCli(cwd: PortablePath, argv: Array<string>): Promise<{exitCode: number | null, stdout: string, stderr: string}> {
+export async function runCli(cwd: PortablePath, argv: Array<string>, options?: Parameters<typeof spawn>[2]): Promise<{exitCode: number | null, stdout: string, stderr: string}> {
   const out: Array<Buffer> = [];
   const err: Array<Buffer> = [];
 
@@ -11,6 +11,7 @@ export async function runCli(cwd: PortablePath, argv: Array<string>): Promise<{e
     const child = spawn(process.execPath, [`--no-warnings`, `-r`, require.resolve(`./recordRequests.js`), require.resolve(`../dist/corepack.js`), ...argv], {
       cwd: npath.fromPortablePath(cwd),
       env: process.env,
+      ...options,
       stdio: `pipe`,
     });
 
diff --git a/tests/main.test.ts b/tests/main.test.ts
index 3e4f8de07..e0f03b09c 100644
--- a/tests/main.test.ts
+++ b/tests/main.test.ts
@@ -29,6 +29,50 @@ it(`should refuse to download a package manager if the hash doesn't match`, asyn
   });
 });
 
+it.failing(`should refuse to download a known package manager from a URL`, async () => {
+  await xfs.mktempPromise(async cwd => {
+    // Package managers known by Corepack cannot be loaded from a URL.
+    await expect(runCli(cwd, [`yarn@https://registry.npmjs.com/yarn/-/yarn-1.22.21.tgz`, `--version`])).resolves.toMatchObject({
+      exitCode: 1,
+      stderr: ``,
+      stdout: /Illegal use of URL for known package manager/,
+    });
+
+    // Unknown package managers can be loaded from a URL.
+    await expect(runCli(cwd, [`corepack@https://registry.npmjs.com/corepack/-/corepack-0.24.1.tgz`, `--version`])).resolves.toMatchObject({
+      exitCode: 0,
+      stderr: ``,
+      stdout: `0.24.1\n`,
+    });
+  });
+});
+
+it.failing(`should refuse to download a known package manager from a URL in package.json`, async () => {
+  await xfs.mktempPromise(async cwd => {
+    // Package managers known by Corepack cannot be loaded from a URL.
+    await xfs.writeJsonPromise(ppath.join(cwd, `package.json` as Filename), {
+      packageManager: `yarn@https://registry.npmjs.com/yarn/-/yarn-1.22.21.tgz`,
+    });
+
+    await expect(runCli(cwd, [`yarn`, `--version`])).resolves.toMatchObject({
+      exitCode: 1,
+      stderr: ``,
+      stdout: /Illegal use of URL for known package manager/,
+    });
+
+    // Unknown package managers can be loaded from a URL.
+    await xfs.writeJsonPromise(ppath.join(cwd, `package.json` as Filename), {
+      packageManager: `corepack@https://registry.npmjs.com/corepack/-/corepack-0.24.1.tgz`,
+    });
+
+    await expect(runCli(cwd, [`corepack`, `--version`])).resolves.toMatchObject({
+      exitCode: 0,
+      stderr: ``,
+      stdout: `0.24.1\n`,
+    });
+  });
+});
+
 it(`should require a version to be specified`, async () => {
   await xfs.mktempPromise(async cwd => {
     await xfs.writeJsonPromise(ppath.join(cwd, `package.json` as Filename), {
@@ -89,7 +133,8 @@ const testedPackageManagers: Array<[string, string] | [string, string, string]>
 for (const [name, version, expectedVersion = version.split(`+`, 1)[0]] of testedPackageManagers) {
   it(`should use the right package manager version for a given project (${name}@${version})`, async () => {
     await xfs.mktempPromise(async cwd => {
-      await expect(runCli(cwd, [`${name}@${version}`, `--version`])).resolves.toMatchObject({
+      const env = {...process.env, COREPACK_ENABLE_URL_VERSION_FOR_KNOWN_PM: `1`};
+      await expect(runCli(cwd, [`${name}@${version}`, `--version`], {env})).resolves.toMatchObject({
         exitCode: 0,
         stderr: ``,
         stdout: `${expectedVersion}\n`,
@@ -99,7 +144,7 @@ for (const [name, version, expectedVersion = version.split(`+`, 1)[0]] of tested
         packageManager: `${name}@${version}`,
       });
 
-      await expect(runCli(cwd, [name, `--version`])).resolves.toMatchObject({
+      await expect(runCli(cwd, [name, `--version`], {env})).resolves.toMatchObject({
         exitCode: 0,
         stderr: ``,
         stdout: `${expectedVersion}\n`,
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