Will a blood clot force Cheney to step down?

The 66-year-old has a history of major health problems, including four heart attacks, and has undergone quadruple-bypass surgery.

He is a driving force in both the war in Iraq and the 'war on terror' and his resignation would be a huge loss to George Bush.

The vice-president is the leader of Washington's neo-conservatives and is believed to be a dominant influence on Mr Bush's military and foreign policies.

He is also believed to be the president's closest political friend.

There has been widespread speculation that if Mr Cheney is unable to complete his term in office through ill health he will be replaced by Secretary of State Condoleezza Rice, another personal friend of Mr Bush.

Mr Cheney called in a doctor yesterday after suffering 'discomfort' in his calf.

'An ultrasound test revealed a deep venous thrombosis or blood clot in his left lower leg,' said an

official report. The vice-president has been put on blood thinning medication and has returned to work.

While the White House is trying to play down the situation, the condition is obviously serious and will again trigger speculation that Mr Cheney could be forced to resign through poor health.

Blood clots that form deep in the legs can become killers if they break off and float into the lungs.

This is called a pulmonary embolism.

Mr Cheney suffered his first heart attack in 1978 at 37. In June

2001, doctors implanted a device that monitors and normalises an irregular heartbeat. Mr Cheney refers to it as his 'pacemaker plus'.

The vice-president is under enormous stress as his policies in Iraq continue to fail and casualty figures mount.

He returned last week from a trip that included unannounced stops in Afghanistan and Pakistan and during which a Taliban suicide bomber tried to kill him.

Since he got back, Mr Cheney has been sharply criticising the opposition Democrats for trying to restrict funds for President Bush's troop build-up in Iraq.

The President relies on him heavily not just to formulate policy but also as an 'attack dog' to take on domestic critics.

There is speculation that the 65 hours Mr Cheney spent in the air aboard Air Force Two over the nine-day trip could have contributed to his thrombosis.

'When you're just sitting there with your legs hanging down for long periods of time, that's what predisposes you to the problem,' said Sean O'Donnell, chief of vascular surgery of Washington Hospital Centre.

The vice-president's travel schedule is likely to be severely curtailed in future.