The prime minister has failed to confirm that he will serve a full third term if he wins the next general election.

Speaking in the Commons, Tony Blair avoided calls by the Conservative leader to spell out his position.

Michael Howard told MPs that he "intends if elected to serve a full term of office" and challenged the prime minister to make the same pledge.

Blair responded that "since I have been prime minister, it is not prime ministers who have gone, it is leaders of the opposition".

But following reports that deputy prime minister John Prescott and chancellor Gordon Brown had discussed the Labour leadership, Howard said: "I can assure the prime minister that no one is plotting against my leadership."

He continued: "But everyone will notice that the prime minister didn't answer the question.

"What everyone will want to know in those circumstance is whether, if they vote Blair they are going to get Brown."

The Tory leader added that Prescott and Brown had stitched the prime minister up "like a kipper".

The session was then interrupted by a major security breach in the Commons, but the prime minister had earlier said that he would be judged on the government's record.

Major's intervention

Earlier in the day, former prime minister John Major had suggested that Tony Blair cannot leave Downing Street while his policies are "in flux".

"I don't myself see how the prime minister could credibly leave at the moment with his policies, I put it kindly, in a state of flux domestically and internationally," Major told BBC Radio 4's Today programme.

Blair would only leave Downing Street if he were ousted by Labour MPs or due to a health or other problem, he insisted.

Major, who was prime minister during the first Gulf war, said that he was more concerned about the situation in Iraq than about Blair's future.