Tony Blair is going through his toughest time yet as Premier

Hartlepool MP and former Cabinet minister Peter Mandelson says Mr Blair would not go "on and on and on and on" beyond the point where he was no longer wanted.

Mr Mandelson told Channel 4 News: "I must say I am less concerned about who is Prime Minister at the end of a third term - as long as it is a Labour Prime Minister - than what that government is doing during its time in office.

"No Prime Minister goes on and on and on and on. Mrs Thatcher said she was going to do so and then found that other people had different ideas.

"I think that for him - he has said this himself before - he is not going to overstay his welcome.

"He is not going to get to the point that, whatever good he is doing for the country, people sort of feel restless and want a change. I don't think he is going to reach that point."

Peter Mandelson: No Prime Minister goes on and on

He added: "Thankfully, or mercifully for him, that point doesn't seem to have been reached yet."

But Mr Blair's agent, John Burton, based in Trimdon in Mr the prime Minister's Sedgefield constituency, said Mr Blair was unlikely to hand over power to Chancellor Gordon Brown.

He said: "Tony is doing fantastically well. He is fit and ready to take on whatever the country wants him to do."

Mr Mandelson's comments would appear to be at odds with another old friend of Mr Blair's - the Constitutional Affairs Secretary Lord Falconer who said last week that he did expect Mr Blair serve a full third term.

Lord Falconer's remarks had angered supporters of Chancellor Gordon Brown who is widely assumed still to nurse ambitions for the top job.

Mr Mandelson stressed he did not expect Mr Blair to give up office any time in the near future, saying: "I would not be getting out handkerchiefs to bid farewell just yet".