The Downing Street switchboard was at meltdown point last week after thousands of callers were duped into phoning the Prime Minister's number.

According to reports, a hoaxer sent an SMS to thousands of mobile phone users with the urgent message: "Hi, mate, phone me ASAP, crap signal in this pub. If I don't answer; ask for Tony who runs the place."

The number given went straight through to the Number 10 switchboard, and a spokeswoman said that 80 per cent of the calls coming in were a result of the prank.

As a result Downing Street was forced to have another private line installed, the number of which has only been released to government officials and trusted callers.

A recorded message has been left on the number which says: "This is the Downing Street switchboard.

"You are receiving this recorded message because of hoax messages in circulation asking the recipient to call this number urgently. If you do not have a reason to call Downing Street, please hang up now."