Tony Blair will undergo a catheter ablation operation which will correct his heart's abnormal rhythm - a condition the Prime Minister has suffered from in the past.
The operation, performed under local anaesthetic, involves inserting a catheter through a vein in the groin. The catheter is then threaded to the heart, where heat is used to destroy short circuits that cause the irregular beats.
On 19 October 2003, the Prime Minister was taken ill at Chequers with the first known occurrence of the condition. He was taken to Hammersmith Hospital in west London where he was given cardioversion, an electric shock which jolts the heart back to a normal rhythm.
Downing Street described his condition at the time as paroxysmal supraventricular tachycardia (SVT) - where the heart beats at twice the normal speed. This is regarded as a serious condition and though frightening, patients can recover from it and never have another attack.
Doctors are likely to do no more than recommend cutting down on tea and coffee and taking life more easily.
Aides insisted that Mr Blair's appetite for the job was undiminished and there was no reason for the condition to recur. Doctors advised that SVT happens to healthy people and once treated, it was compatible with a completely normal quality and quantity of life.
But specialists pointed out that cardioversion was not the usual treatment for SVT and suggested the Prime Minister was more likely to be suffering from atrial fibrillation. The condition has now recurred making today's second bout of treatment necessary.
The disclosure by Bill Clinton, former US president, that he had "known for years" about the Tony Blair's cardiac arrhythmia was regarded as highly embarrassing by Downing Street.
At 50, Mr Blair is considered young to require treatment for a heart condition. Most men who undergo this treatment are in their 60s and 70s or older. The Prime Minister retained his boyish looks for longer than is usual and has taken care of himself during his years in No 10 with regular sessions in his private gym.
He doesn't smoke, drinks moderately, eats healthily and is not overweight. One factor that may have made him prone to the problem is his genetic inheritance. His father Leo Blair suffered a near fatal stroke at the exceptionally young age of 39.
What must also be on his mind as he prepares for this operation is that it was a heart attack which carried off his predecessor John Smith, the former Labour leader, creating the opening which he took.
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