He still has a course of radiotherapy to come.But Mr Campbell, MP for North East Fife since 1987, says he would not have run for the deputy leadership if he did not think he was up to it “The treatment has been successful,” he says. “The doctors are extremely pleased and I am going to have some radiotherapy as an insurance.”The cancer was discovered by chance last autumn after leg pain caused Mr Campbell to start limping. “I thought I needed a hip replacement, and the surgeon made a proper investigation and discovered it was a tumour,” he says. When news of his illness emerged, he received hundreds of letters of support. Colleagues were amazed to see him pop up on frequent television interviews. “I successfully had the last [chemotherapy session] last week,” he says. “All the indications are that the bone has regenerated and is very much better than anyone had expected.

I am very lucky.”Mr Campbell’s track record is an indication of his staying power. At Glasgow University, where he studied law alongside Donald Dewar, the former Scottish secretary and first minister, he became president of the union. He also excelled in athletics and between 1967 and 1974 was Britain’s fastest man, holding the UK 100m record. He competed for Britain at the Olympics and Commonwealth games and captained the UK athletics team.These days, Mr Campbell’s focus is on Iraq, and how to slow the sprint towards war. The Liberal Democrat’s foremost Commons statesman has been called the “most sensible” voice in Parliament on the war, and as a robust supporter of military action in Kosovo and Afghanistan he has more credibility than many other critics of the Government. “Before force can be justified as a last resort, you have to be satisfied that you have exhausted every other political or diplomatic alternative,” he says. Mr Campbell believes Saturday’s million-strong protest against the war makes it more difficult for Tony Blair to pursue military action.

But he does not doubt the sincerity of the Prime Minister’s position and is scathing about those who accuse Mr Blair of being President George Bush’s “poodle”. He says: “In my view, that language fails to take proper account of the fact that the Prime Minister, for whatever reason, really believes in the course of action he is engaged in. I know him well enough to believe this is his independent judgement.”But for Mr Blair to “go it alone” with Mr Bush and back a strike on Iraq without Nato or UN consensus would be “an enormous gamble”, Mr Campbell says, adding: “He would be gambling everything on total and speedy success.”The Scot ­ who for many years has been a member of the UK’s delegation to the North Atlantic Assembly, Nato’s Parliamentary Forum ­ warns that a war with Saddam Hussein may not be speedy or simple. If President Saddam was cornered, he might launch chemical or biological attacks against neighbouring nations such as Israel.