As long ago as 1983 Michael Foot was written off in much of the media as a frail old man only just capable of walking his dog around Hampstead Heath. Now, while many of his tormentors in the newspapers have either retired or expired, Foot, and indeed his dog, are as active as ever. At the age of 85 he has just completed his latest book, Dr Strangelove, I Presume. Dizzy, the most photographed dog in the early 1980s, sits by the sofa in the study of Foot's Hampstead house: the two of them have just returned from a walk on the heath.

After the 1983 election defeat and his resignation as Labour leader, Foot wrote two political books. Another Heart and Other Pulses was his account of that catastrophic campaign in which Labour was nearly beaten into third place by the SDP/Liberal Alliance. It conveyed the degree of media animosity towards Foot in particular and Labour in general. Subsequently he wrote a series of essays, Loyalists and Loners, focusing on the main political personalities of the 1970s and early 1980s. With new Labour moving on to the political stage, Foot chose to write more about his literary heroes. His most recent book was a biography of H G Wells.

But evidently Foot found it impossible to bite his political lip indefinitely. One of the leaders of the Aldermaston march and star speaker at the CND rallies of the early 1980s when he was leader of the Labour Party, he has now written a passionate book calling for global nuclear disarmament and warning of catastrophe if the world does not rid itself of all nuclear weapons. Dr Strangelove goes on to accuse western governments, including the current British one, of dangerous complacency.

Sitting at his desk, almost dwarfed by various publications and newspapers, he explains why his long obsession with the dangers of nuclear weaponry has finally become the subject of a book. "India is the simple answer. The more I think about it the more outraged I am about what has happened to India and how neglectful we have been in this country and blind to what is happening there."

He is referring to the recent development of nuclear weapons in India and in Pakistan, as the two countries embark on their own distinct nuclear arms race. "Our prejudice helped to get the arms race going and helped to push India and Pakistan into it. Up to Rajiv Gandhi's time, the Indians wanted to follow an entirely different policy. If his ideas had been adopted we could have avoided this terrible catastrophe in India. Twelve years ago, Rajiv proposed a plan which could have stopped the whole of this nonsense going ahead, but he warned that India couldn't hold back if other countries were allowed to develop weapons. Time and again Indian leaders said that we don't want this nuclear programme, but if Pakistan gets one we will have no choice. We in Britain have no answer to that because we ourselves used that argument to get these damned weapons. We said that if one side had them we needed them, too."

Foot was inspired to write a book after a visit to Delhi last year for the bi-annual Indira Gandhi seminar. He has been a regular attender since the assassination of the Indian prime minister, who was a personal friend. At the seminar the former US defence secretary Robert McNamara argued for the abolition of all nuclear weapons. "He was talking as we did on the Aldermaston march. He explained how these weapons are so hideous and uncontrollable. This was not an outlandish English rebel like me speaking, but the chap who's seen it from the top in America."

Unilateral nuclear disarmament was a fashionable, if electorally unpopular, cause in the early 1980s. Many of the current cabinet, including Tony Blair, were members of CND. While they regard their membership as a political embarrassment now, Foot has remained unswerving in his commitment. "No doubt the issue has subsided since the 1980s. Of course, the Labour Party then led the protest against the cruise missiles, and they were good demonstrations, by the way, and although we didn't get rid of the missiles at once, we did so eventually and I think all the people who protested in different forms, starting with the women at Greenham Common, had an effect."

So why has there been a decline in the protest movement? "The main reason is the agreements between Reagan and Gorbachev which, although they did not abolish the weapons, looked forward to such a possibility. By the way, Gorbachev persuaded Reagan on this. Reagan had gleams of intelligence on the subject and when he looked at it he concluded that he could never press the damn button. Since then there's been a tragic decline in the recognition that the danger of catastrophe remains as long as the weapons remain."

Foot's sentences are long, but they are not the consequence of the tortured pragmatism which affects political interviewees today. Instead one point triggers another in his mind, demanding immediate articulation. Why bother with a full stop? His never-ending sentences are motivated by commitment rather than the need to disguise a lack of it. There are, though, some problems. His call for the abolition of all nuclear weapons seems so distant and unachievable that it risks being ignored by more practical politicians. What is more, Labour, under his leadership and his advocacy of unilateral nuclear disarmament, lost heavily in the 1983 election.

"At the time of my leadership people said that we were trying to get rid of the weapons and nothing else. Well, we could have got rid of the weapons unilaterally because they serve no defensive purpose, but actually we were trying to get international agreements. Now the present Labour Party has repudiated the policy of giving up the weapons unilaterally. I understand why they've done it. I don't think it was necessary, but they believe the old policy was so misrepresented it had to change. But that does not mean we should have as negative a defence policy as the one we are pursuing at the moment."

So far, Foot has refrained from criticising the government, although in private he has a range of concerns. Mindful always of the awful time he had at the hands of internal critics when he was leader, he knows the value of loyalty. Nonetheless, in this interview, he was highly critical of the government's defence policies in general and of its deference to the United States in particular. He opposes the bombing of Iraq, arguing that "it was not the best way to get rid of Saddam. Indeed it looks as if it could have had the opposite effect and made his position more secure".

But he feels even more strongly about other American military offensives and Tony Blair's support for them. "The way Clinton bombed Khartoum and the terrorists in Afghanistan was a scandalous misuse of power and it was quite improper of the British government to give its support. America and Britain argued there was a danger of chemical gases being developed, but the bombs risked blowing these places to pieces so nobody would know for sure. If chemical gases were the immediate threat, inspection teams could have gone out there. In America many people are questioning Clinton's claims about the whole damned thing. The way Clinton used his powers on these matters was very grave and I think at one stage our government alone was prepared to go along with it. That was quite wrong. The government should be persuading Clinton to change his approach, and if we can't persuade them I don't know who can."

In terms of the British government's own agenda, Foot singles out the Strategic Defence Review for criticism. "We are committed to keeping nuclear warheads sitting around on medium alert for the next 30 years and unless other countries reduce theirs we are not going to make any further cuts. In my opinion that is nowhere near good enough to deal with the international situation. Instead we should be offering our expertise to help dismantle these horrific weapons, and offer help where there's need for inspections where these damned things have to be dismantled. Robin Cook was talking about this before the election. I think he should be talking about it now. Instead, whenever a crisis arises it looks as if we are prepared to do whatever the Americans say."

Typically, Foot's book is sprinkled with references to old friends who have been prominent nuclear disarmers. The journalist James Cameron is quoted at length. A brilliant essay Foot wrote on A J P Taylor is reprinted. But the repertoire of dazzling names that Foot tends to call up for support are either dead or no longer active in politics. Who are the young charismatic figures to move the cause on? Foot offers no new names, but glories in the arguments of old friends, which he regards as more relevant than ever. "I'm pleased you mentioned Alan Taylor. He put the case against the deterrent in its historic context. He illustrated how the deterrent failed to stop the two great wars in this century and how it won't stop the next one. During the last election campaign, wherever I went, north, south, east and west, I met chaps who had been pupils of Alan's. I don't say they've all adopted Alan's views on the subject - I wish they had - but they are applying their minds to this issue with absolute integrity, inspired by Alan. There are a lot of good MPs arguing the case. They have not vanished from the face of the earth even if the whips would prefer them to."

Foot has written at length about his father, his friends, his political colleagues and his literary heroes. He rarely gives much away about himself. I ask him, now that his book is finished, what he is reading at the moment. His answer soon reverts to the issue in hand. "I've gone back to John Milton. He was the greatest campaigner of them all, you know, and I'm sure if he was alive today what he would say about the bomb and the bombers would be absolutely unforgettable."

I leave on a mild winter's day without noticing I have left my very new Labour Nicole Farhi coat behind. Two days later I remember that I must have left it at Foot's house. I phone him to check. "Ah yes, the coat. My wife, Jill, said to me when she saw it that I had got myself a decent coat at last. You'll be lucky to get it back, you know."

Foot's humour, and appetite for politics, both tested to the limit in the build-up to the 1983 election, are still intact more than 15 years later.

"Dr Strangelove, I Presume" is published by Gollancz, £16.99