Plans being considered by the Commons security commission include allowing bodyguards into the chamber to protect the prime minister during his appearances at the despatch box.

It follows last week's purple flour bomb attack by members of the campaign group Fathers 4 Justice.

"It is up to Downing Street but it is one of the options," one official told the Independent.

Ministers have dismissed suggestions that the timing of prime minister's questions could be changed to foil any assassination plans.

"It wouldn't take killers five minutes to find out when it was going to happen," said a Westminster source.

Meanwhile, Tony Blair is today expected to end questions over his leadership, by pledging to fight for a full third term in office.

Last night, a Downing Street spokesman said that if asked about it in his monthly press conference, the prime minister will refer to comments he made on the eve of the Labour Party conference last year.

"Well, if you stand for election, you stand for a full term. That's clear, of course it is," he said then.

It follows claims from Michael Howard that those voting for Blair and Labour at the next general election would get Gordon Brown as prime minister.