A hoax caller who was put through to David Cameron's mobile phone apologised to the prime minister for "waking him up" in the middle of the morning while the Tory leader was on a family walk.
Cameron revealed during a speech in Eastleigh that he ended the conservation as soon it was clear the caller was a hoaxer and stressed that he did not disclose any sensitive information.
"My BlackBerry went in my pocket, I answered it and it claimed to be a conference call established between the head of GCHQ [Robert Hannigan] and some of the staff in my office," the prime minister explained.
"A voice came through, a voice I didn't recognise, the voice said that he was sorry to wake me up, which I thought was strange as it was 11 o'clock in the morning, and so I rapidly asked 'who is this?'.
"To which the answer came 'it is a hoax call' and so I pushed the red button on the BlackBerry, which ended the call."
A man, purporting to be the caller, said he made a "complete monkeys out of GCHQ".
"I got the mobile number of a director. What's more, I'm off my face on booze and cocaine. I had some spliff too. I've been up all night, I'm utterly wasted – hilarious," he told The Sun.
He added: "What's really funny is that GCHQ believe the word of someone so smashed they can hardly string a sentence together."
Security procedures at Downing Street and GCHQ are now being reviewed after the incident.
A government spokeswoman said a notice had also "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 spokeswoman said.
"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."