Rogue text messages have left the Downing Street switchboard jammed by hoax callers.

Recipients of the message are urged to call the number urgently, but are unaware that they will reach Number 10.

Such callers accounted for 80 per cent of calls to the switchboard last week, leading to a special recorded message being set up.

"The increase in calls has put enormous pressure on our small team of switchboard operators who have a very difficult and responsible job to do," a Downing Street spokesman said.