A hoax caller who got through to spy agency GCHQ was "off his face on booze and cocaine" and has threatened to do it again.
The man, believed to be in his twenties, bragged to The Sun newspaper about his apparent breach of security saying he "made complete monkeys out of GCHQ" and said he would do it again because "it was so easy".
He managed to obtain a mobile phone number for GCHQ director Robert Hannigan.
A second call was also made later to Prime Minister David Cameron from a man – not known whether it was the same prankster - claiming to be the GCHQ director. The phone call was "quite brief" when it became clear it was a prank.
Downing Street and GCHQ are now reviewing their security following the two breaches.
A Government spokesman 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 phone 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.
"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."
A man claiming to be the hoaxer called The Sun last night and said: "I've just made 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 added: "What's really funny is that GCHQ believe the word of someone so smashed they can hardly string a sentence together."
Former Prime Minister Tony Blair also fell for a prank call from a DJ at Capital Radio radio station who pretended to then be Tory leader William Hague.