Tony Blair will today go into hospital for treatment after suffering a recurrence of his irregular heart beat.

The Prime Minister said last night that despite the procedure, he would still seek a third term and, if elected, serve it in full.

However, Mr Blair, 51, went on to say he would then stand down as PM and not seek a fourth term.

Facing renewed questions about his future, Mr Blair said he wanted to give people certainty.

"The decision, of course, is one for the country but if elected I would serve a full term, but I would not then stand for a fourth term," he said.

"I want to put myself forward for a third term and if elected I would serve that full term.

"But then I think it is sensible to say I would not then stand for a further term beyond that."

Mr Blair will be sedated during the two-and-a-half hour procedure, called a catheter ablation.

It was needed after a recurrence this summer of the heart condition he suffered last year, supraventricular tachycardia.

Downing Street played down the seriousness of the procedure and refused to say where it would be performed or when Mr Blair had again become ill.

The PM will spend tonight in hospital and rest over the weekend before returning to normal duties on Monday and continuing with a visit to Africa on Tuesday, a spokesman said.

Deputy Prime Minister John Prescott will be in charge while Mr Blair is treated.

Concerns over Mr Blair's health were first raised when he suffered a heart scare nearly a year ago.

The Prime Minister was diagnosed with supraventricular tachycardia, or SVT, which is often described as an electrical short-circuiting of the heart.

Mr Blair spent five hours in hospital on October 19 after complaining of chest pains while staying at Chequers.

Doctors at London's Hammersmith Hospital gave him a cardioversion, an electric shock, to correct the irregular heartbeat.

The Prime Minister said afterwards that it was the first time he had suffered a heart problem.

Last night, speaking to the BBC's political editor Andrew Marr, Mr Blair said his condition was not alarming.

''It's something that you should get fixed," he said. ''It's a routine procedure.

''I've had it for the last couple of months and it's not impeded me doing my work and feeling fine but it is as well to get it done.''

Asked whether the announcement was good or bad for the Chancellor, Mr Blair said: ''I have always said I think Gordon Brown would make a superb Prime Minister of the country. I really do. And I'm not saying what can happen in time to come.

''I am simply saying I want to take the party into the election. I think the party wants me to take the party into the election and if you stand you have to stand for the full term.''

A spokesman for Mr Brown, who is in Washington, said:"His thoughts will be with the Prime Minister throughout the weekend. This evening he sent Tony a personal message of support.''

Tory leader Michael Howard said: ''I am sorry to hear about the Prime Minister's ill health. I wish him well and a speedy recovery.''

John Burton, Mr Blair's constituency agent in Sedgefield, said: ''Over the last few weeks that I have known about it, he has been to the gym and he has been playing tennis. It is just routine.

''Of course, anything to do with the heart is of concern, but this has been planned.''

According to the British Heart Foundation, supra-ventricular tachycardia is caused by rapid electrical activity in the heart, but is not considered serious.

The most common symptom is awareness of a particularly fast heartbeat. This may be accompanied by shortness of breath, dizziness or fainting.