Security procedures are being reviewed at Downing Street after a hoax caller pretending to be the head of GCHQ managed to get through to David Cameron on Sunday.
Cameron spoke to the imposter, who was claiming to be the GCHQ director, Robert Hannigan, but ended the call when he realised he was being tricked.
According to No 10, no sensitive information was disclosed during the conversation between the men, which was described as “quite brief”.
In a separate incident, a caller rang GCHQ and managed to obtain Hannigan’s mobile phone number.
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.