INVALID_APP_CREDENTIAL for GoogleIdentityToolkit API

Question

I am using firebase phone authentication feature for web. I have implemented it by using javascript sdk. It is working unexpectedly from last few days. When I am requesting to send verification code after entering mobile number and captcha verification it is returning an error

Error: The phone verification request contains an invalid application verifier. The reCAPTCHA token response is either invalid or expired.

When I checked for api calling in browser's network tab I have seen that below api is getting called https://www.googleapis.com/identitytoolkit/v3/relyingparty/sendVerificationCode?key=AIz********************

And it is returning

{
  "error": {
    "code": 400,
    "message": "INVALID_APP_CREDENTIAL",
    "errors": [
      {
        "message": "INVALID_APP_CREDENTIAL",
        "domain": "global",
        "reason": "invalid"
      }
    ]
  }
}

I am using below two JS plugins https://www.gstatic.com/firebasejs/6.3.3/firebase-app.js https://www.gstatic.com/firebasejs/6.3.3/firebase-auth.js

Searched on internet but nothing relevant found.