DOWNING Street was last night playing down concerns over the health of Tony Blair after the Prime Minister was treated in hospital for an irregular heartbeat.

The 50-year-old Prime Minister spent almost five hours with doctors after being taken ill at Chequers.

He was taken to the nearby Stoke Mandeville Hospital - although not by ambulance - but was moved to Hammersmith hospital in London as an acute admission as it has a specialist coronary care unit.

Mr Blair, a father-of-four, was given a cardio-version, the normal treatment for the condition, before returning to Downing Street.

Doctors have told him to rest for 24 hours and he has pulled out of plans to give a statement in the House of Commons today on the European summit.

Foreign Secretary Jack Straw will deliver that in his place but Mr Blair will hold meetings throughout the day, before returning to work full-time tomorrow.

A spokesman for Downing Street said: ''This morning, the Prime Minister was feeling under the weather and went to Stoke Mandeville hospital where they advised him to go to the Hammersmith.

''There it was established he had an irregular heartbeat and a cardio-version was administered to regulate it.

''This was completely successful. He was in hospital for four to five hours and is now back at Number 10. The hospital says this is a relatively common condition and is easily treated.

''He has suffered no damage and he is fine. There is no reason why this should reoccur. They have advised him to rest for 24 hours."

Tory leader Iain Duncan Smith was quick to send his best wishes to Mr Blair.

In a statement, he said: ''I was very sorry to hear that the Prime Minister was taken ill today and was briefly admitted to hospital.

''I very much hope he makes a swift recovery and I send him and his family all best wishes at this difficult time.''

It is the first time Mr Blair has been known to be ill since becoming Prime Minister.

He has been under strain in recent months, particularly over the war in Iraq and the recent Hutton Inquiry into Dr David Kelly's death. He has also faced difficulties with backbenchers and the trials of Labour's recent party conference.

Last night, Mr Blair's agent in his Sedgefield constituency, John Burton, said: "I've talked to Cherie and he's absolutely fine. He's doing great.

"We are close family friends, we just asked after Tony. We were talking about him as just Tony, not as the Prime Minister."

Mr Blair prides himself on being in shape.

Despite his hectic and stressful schedule, the father-of-four finds time to play tennis at Chequers, goes swimming, and enjoys a spot of football.

In an interview for his 50th birthday earlier this year, he was described as weighing just under 13 stone, less than he did a decade years ago.

''I feel great, physically. I do more exercise today than I've done since I was at school,'' he told Saga magazine.

''I pay more attention to looking after myself, I watch my diet a bit. But really I find it's exercise that's fantastically helpful for coping with stress.''

Although looking somewhat older than when he first came to power, he has kept good health.

Yesterday's diagnosis of an irregular heartbeat is the first known problem since becoming Prime Minister in 1997.

Mr Blair's elderly father Leo - after whom he named his fourth child - was left frail after a second stroke two years ago. He was only 39 when he suffered his first stroke, and took three years to recover his speech.

Mr Blair's mother, Hazel, died in 1975 after a five year battle against thyroid cancer.

Asked by Saga if his genetic links made him concerned for his own health, Mr Blair admitted: "Yes, I suppose so."

Last night, Mr Burton said people in the Prime Minister's Sedgefield constituency, in County Durham, were rallying round and wishing Mr Blair a speedy recovery.

"People in the constituency have been in touch - they're his friends and they're naturally concerned. But now they're thrilled to bits that everything is fine."

One of the Blairs' close family friends, Father John Caden, the 80-year-old Roman Catholic parish priest of Sedgefield, said prayers would be said for Mr Blair and his family at this morning's service at St John Fisher Church.

Fr Caden has been a friend of the Blairs for two decades.

He said: "It's probably just a warning, so we'll pray that he recovers and is able to continue as Prime Minister."