British Prime Minister David Cameron has insisted that "no harm was done" by a hoaxer who called him on his mobile phone pretending to be the head of spy agency GCHQ.

He spoke as it emerged that before the prank, the man thought to be responsible, had bragged that he had also called GCHQ despite being "off my face" on alcohol and drugs.

Both Cameron's Downing Street office and GCHQ, the British intelligence agency which monitors communications, are now reviewing their security procedures, officials said on Monday.

Cameron revealed he was out walking with his family on Sunday morning when his phone rang with what appeared to be a conference call with GCHQ director Robert Hannigan.

He then heard the voice of a man he did not recognise, who apologised for waking him up - despite it being 11am. He asked who was speaking, and the man said it was a hoax call.

Cameron said he promptly hung up, adding: "No harm was done, no national security was breached.

"But it is important when these things happen to make sure we do everything we can to put in place systems to weed out hoax calls."

Earlier, The Sun newspaper revealed it had been called by a man thought to be the prankster, who bragged about how he had earlier also managed to speak to Hannigan on the phone.

"I've just made complete monkeys out of GCHQ. I've got the mobile number of the director," he said.

"What's more, I am off my face on booze and cocaine. I had some spliffs too. I've been up all night. I'm utterly wasted. Hilarious."

He said he could "hardly string a sentence together" but added: "I'm definitely going to do it again. It was so easy."

A government spokeswoman confirmed that a hoax caller had obtained Hannigan's phone number but said that line was "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," she said, adding: "In neither instance was sensitive information disclosed".