diff --git a/src/authentication-method.ts b/src/authentication-method.ts
index 37229a4..42fc0e0 100644
--- a/src/authentication-method.ts
+++ b/src/authentication-method.ts
@@ -6,11 +6,12 @@ import type { AuthenticatorCredentials } from "./user";
 export async function authenticationMethodFromEnv(env: Env) {
   if (env.JWT_REGISTRY_TOKENS_PUBLIC_KEY) {
     return await newRegistryTokens(env.JWT_REGISTRY_TOKENS_PUBLIC_KEY);
-  } else if (env.USERNAME && env.PASSWORD) {
-    const credentials: AuthenticatorCredentials[] = [
-      { username: env.USERNAME, password: env.PASSWORD, capabilities: ["pull", "push"] }
-    ];
+  } else if ((env.USERNAME && env.PASSWORD) || (env.READONLY_USERNAME && env.READONLY_PASSWORD)) {
+    const credentials: AuthenticatorCredentials[] = [];
 
+    if (env.USERNAME && env.PASSWORD) {
+      credentials.push({ username: env.USERNAME, password: env.PASSWORD, capabilities: ["pull", "push"] });
+    }
     if (env.READONLY_USERNAME && env.READONLY_PASSWORD) {
       credentials.push({ username: env.READONLY_USERNAME, password: env.READONLY_PASSWORD, capabilities: ["pull"] });
     }
@@ -18,7 +19,7 @@ export async function authenticationMethodFromEnv(env: Env) {
     return new UserAuthenticator(credentials);
   }
 
-  console.error("Either env.JWT_REGISTRY_TOKENS_PUBLIC_KEY must be set or both env.USERNAME, env.PASSWORD must be set.");
+  console.error("Either env.JWT_REGISTRY_TOKENS_PUBLIC_KEY must be set or both env.USERNAME, env.PASSWORD must be set or both env.READONLY_USERNAME, env.READONLY_PASSWORD must be set.");
 
   // invalid configuration
   return undefined;
diff --git a/test/index.test.ts b/test/index.test.ts
index 5c65e35..09d7472 100644
--- a/test/index.test.ts
+++ b/test/index.test.ts
@@ -466,6 +466,30 @@ describe("http client", () => {
 
     expect("exists" in res && res.exists).toBe(false);
   });
+
+  test("test manifest exists with readonly", async () => {
+    envBindings = { ...bindings };
+    envBindings.JWT_REGISTRY_TOKENS_PUBLIC_KEY = "";
+    envBindings.PASSWORD = "";
+    envBindings.USERNAME = "";
+    envBindings.READONLY_PASSWORD = "world";
+    envBindings.READONLY_USERNAME = "hello";
+    envBindings.REGISTRIES_JSON = undefined;
+    global.fetch = async function (r: RequestInfo): Promise<Response> {
+      return fetch(new Request(r));
+    };
+    const client = new RegistryHTTPClient(envBindings, {
+      registry: "https://localhost",
+      password_env: "PASSWORD",
+      username: "hello",
+    });
+    const res = await client.manifestExists("namespace/hello", "latest");
+    if ("response" in res) {
+      expect(await res.response.json()).toEqual({ status: res.response.status });
+    }
+
+    expect("exists" in res && res.exists).toBe(false);
+  });
 });
 
 describe("push and catalog", () => {