The caller claimed to be GCHQ’s director Robert Hannigan. The prime minister ended the call after a “brief” conversation.

Stefan Rousseau/PA Wire

The caller claimed to be GCHQ director Robert Hannigan. Cameron ended the call when it became clear it was a hoax and no sensitive information was disclosed, Downing Street said.

The conversation was said to have been "quite brief" before the hoax was discovered.

A separate hoax call was made to GCHQ where Hannigan's mobile number was given out to the caller.

This number is thought to have been for an unclassified phone rather than a secure line used for sensitive communications.

A Government spokeswoman said: "Following two hoax calls to government departments today, a notice has gone out to all departments to be on the alert for such calls.

"In the first instance, a call was made to GCHQ which resulted in the disclosure of a mobile phone number for the director.

"The mobile number provided is never used for calls involving classified information. In the second instance, a hoax caller claiming to be the GCHQ director was connected to the prime minister.

"The prime minister ended the call when it became clear it was a hoax. In neither instance was sensitive information disclosed.

"Both GCHQ and Number 10 take security seriously and both are currently reviewing procedures following these hoax calls to ensure that the government learns any lessons from this incident."