A hoax caller has been through to David Cameron's phone after claiming to be the director of eavesdropping agency GCHQ.
The Prime Minister ended the call when it became clear it was a hoax and no sensitive information was disclosed, Downing Street said.
Security procedures are being reviewed at both No 10 and GCHQ, where a mobile phone number for director Robert Hannigan was disclosed to the caller.
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."
It is understood that the mobile number given out for Mr Hannigan was for an unclassified phone rather than one of the secure lines used for sensitive communications.
The call to the Prime Minister was made to an official mobile but the conversation was understood to have been "quite brief" before the hoax was discovered.
The incident comes two years after two Australian DJs, posing as The Queen and Prince Charles be put through to staff at the London hospital, where the Duchess of Cambridge was being treated for acute morning sickness.
Mel Greig and Michael Christian were put through to night sister Jacintha Saldanha, 46, who then passed them to a colleague treating the senior royal who gave detail of the Duchess's condition.
She later committed suicide after sending emails to colleagues in which she was clearly distressed about her inadvertent role.
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