A drunk caller pretending to be Britain's top spy chief successfully breached procedures to speak to Prime Minister David Cameron, prompting authorities to review security measures to weed out such calls in future.
The prime minister ended the call when it became clear that it was a hoax and no sensitive information was disclosed, the Downing Street said.
Cameron said he was out walking with his family this morning when his mobile phone rang with what appeared to be a conference call with Government Communications Headquarters (GCHQ) director Robert Hannigan.
He said he did not recognise the voice of the man, who apologised for waking him up - although it was around 11:00 am already.
On being asked who he was, the man said it was a hoax call following which Cameron promptly hung up, the PM said.
"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," Cameron said.
However, the giggling prankster - who claimed to be Hannigan - telephoned The Sun tabloid and said he will "definitely do it again" as it was "so easy" to make the hoax call.
The man allegedly rang up the newspaper to boast "I've just made monkeys out of GCHQ", minutes after his phone conversation with Cameron.
It was one of two hoax calls reported yesterday.
An earlier call was made to GCHQ during which Hannigan's mobile phone number was disclosed, a government spokeswoman said in a statement.
It is understood that the mobile number given out for Hannigan was for an unclassified phone rather than one of the secure lines used for sensitive communications.
He is understood to have said Hannigan was needed at a security meeting but was not answering his "usual number".
"In neither instance was sensitive information disclosed," the spokeswoman said.
The man who claims to be behind the hoax phone calls has said he was "off (his) face on booze and cocaine" at the time.
He told the newspaper the situation was "hilarious" and added: "I'm definitely going to do it again. It was so easy."
Authorities are now reviewing procedures, both at the PM House, 10 Downing Street and GCHQ - which monitors communications, to help ensure that such calls are not patched through again, the spokeswoman said.
"Both GCHQ and No 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," he said.
The call to Cameron was made to an official mobile but the conversation was understood to have been "quite brief" before the hoax was discovered.