Ever since Alan Maclean was recalled as a young man from his diplomatic post in New York, walking, as he puts it, into a “world-class scandal”, he has lived under the shadow of treachery. Arriving at London Airport on a cold morning in 1951, he was hurried into a Daimler by anonymous bureaucrats and told that his brother, Donald Maclean, was suspected of being a Soviet spy and had disappeared with Guy Burgess. The pair later reappeared in Moscow, having escaped on a cross-Channel steamer from Southampton to St Malo, the beginning of their long journey into ignominy.

The event was the defining moment of Alan Maclean’s life. “I knew as soon as I was called home,” he says, “that any hope I had of a diplomatic career was over and that I would have to look for something else to do.”

Being bright and well-connected, he quickly found work in publishing, surprisingly un concerned at being deprived of his career of choice by a brother whose deceit and duplicity he refuses to condemn. “Why should I condemn him?” he asks, peaceably enough. “I’m not a political person, and if you love somebody it doesn’t matter what they do.” He pauses, sips iced water. “No, of course, it matters. What I meant is that you can’t stop loving someone just because they have done something absolutely frightful.”

Reclining in his chair, elegantly smoking a long, thin menthol cigarette, Maclean is the model of a certain type of a Englishman slowly disappearing from public life: the urbane amateur. The self-consciously preposterous title of his memoirs offers a flavour of the man. “Once,” he says, explaining the background, “when I was working at Macmillan publishers, we jokingly tried to think of the most boring title for an autobiography. I came up with No, I Tell a Lie, It Was the Tuesday , and made a private note to use it if I ever wrote my autobiography.”

His father, Sir Donald, was a distinguished solicitor and former Liberal Cabinet minister, and Alan’s early years were spent in affluent seclusion in north Cornwall. Donald, whom he adored, was 12 years his senior.

In his jaunty memoir, Maclean writes of Donald with wary tenderness, describing his brother as “glamorously distant”, a tall, athletic, non-conformist.

Donald’s political radicalism was apparent even during his time at Gresham’s School in Norfolk, where he read Marx and Hegel. “I think his headmaster, though pleased that Donald was bright and responsible enough to take his own political line, thought he would grow out of it.”

He never did, of course. At Cambridge, where he gained a starred first, and all through his years at the Foreign Office and at the British Embassy in Washington, Donald spun a web of elaborate deceit. As a diplomat, he was meticulous and conscientious, a rigid stickler for the Official Secrets Act. But he was also part of an intricate spy network, with branches in the United States, Canada and Britain, as well as being prone to night rages and drunkenness.

Reflecting on his brother’s betrayal, Maclean says: “I can’t help but look back at what he did with anything but detachment. I was absolutely devoted to him. He was terribly good to me when I was a child, especially when I was so miserable as a schoolboy at Stowe. I remember, in particular, Donald saying to me that if things ever got too bad at school, I should let him know and he would come and get me out.”

That expression of reassurance, he says, offered an “escape route” from a school, the dormitories of which were patrolled by a “bullying, alcoholic” housemaster.

Maclean’s childhood, blighted by the death of his father when he was seven, divides into the “happy years” of his Cornish idyll, and those spent in Kensington, where his mother opened a knitwear shop, and at Stowe. His London years were set against the political turmoil of the Civil War in Spain and the gathering clouds of another war - the years when many young Oxbridge intellectuals were drawn to the messianic socialism of Marx and Lenin. Maclean says: “In this respect, Donald was shaped by the attitudes and events of his time. He became more committed after Spain.”

Of the Cambridge spies, Maclean agrees that Kim Philby and the debauched Guy Burgess, driven by a reckless fear of boredom, derived pleasure from a life of treachery. He is less sure, however, about his brother. “Being a spy isn’t something my brother would have entered into lightly. It is a terrible way to live, just awful.” Maclean clearly understands that there is no one lonelier than the double agent. Addicted to secrecy and loyal only to himself, he lives in a condition of perpetual watchfulness.

After Donald defected to Moscow, Maclean feared that he would never hear from him again. Then, in 1956, five years after his disappearance and following Khrushchev’s famous denounciation of Stalin at the 20th Party Congress, he received a telegram. “It was brief. He said that he was well, and hoped that the family was well, too. He asked for the address of my mother, who had never lost faith in him. He expressed no regret, and had faith that the Soviet Union would get better.”

Harold Macmillan, under whom Alan worked briefly at the publishers Macmillan, famously described the patrician Donald as a “class traitor”. Discussing the Cambridge spies in the Commons, Macmillan said: “Our Foreign Office regards this case as a personal wound, as when something of the kind strikes at a family, or a ship, or a regiment.”

Maclean loyally defends his brother. “If Macmillan was implying that Donald had an obligation to his class rather than to what he thought to be politically and humanely fair, then he was wrong. The word traitor also implies that my brother felt, in some way, excluded or shut out, that he wanted to take revenge. What motivated him was a sense of injustice. He acted not for money or from hate, but from conviction. I would have preferred it if he hadn’t done what he did, but he was a political animal.”

One of the most vivid episodes in No, I Tell a Lie concerns Maclean’s journey to visit his brother in Moscow. It was January 1983 and proved to be their only meeting for more than 30 years - Donald died a month later. Frail, stooped and walking with a stick, Donald met his brother at the aiport, pulling him gently into an embrace. He expressed surprise at Alan’s grey hair, as though it had violated the image he carried of his brother in his head.

“Donald looked not too bad, a bit grey in the airport lights. We held hands rather shyly like children. It was going to be all right.” They talked “greedily” that night, but mostly about their childhoods. “Donald said very little about what had happened to him.”

No , I Tell a Lie is an unconventional memoir; Maclean delights in its idiosyncrasy, in what is left out. There is nothing, for instance, about his wife of 31 years, or their two sons, the younger of whom, Dan, died at the age of ten; nothing about his inner life or his convictions.

Repeatedly, he says that he has “no political beliefs”, as if he were deliberately positioning himself against the hard ideology of his brother. He is cannily interesting, though, on the treatment he received for alcoholism in the early 1960s.

There is something inscrutable about Maclean that is hard to account for. Intelligent, charming but shrewdly circumspect, he leaves much unsaid. His voice drops as he talks about his family, but is animated when discussing his public years in publishing. Ask him, for instance, about his elder son, Ben, 29, and he becomes vague. “I’m not quite sure what he does; I think he paints people’s houses and does a bit of research.”

Of his wife, Robin, whom he met at Macmillan, he is equally cryptic. “She used to work as my secretary. We had a whirlwind seven-year romance and then got married.” When the photographer arrives, Alan says: “Oh, look. Here comes the hangman.”

He begins to chuckle. “Look, I feel it’s much better to write and talk about those who are dead. You can’t hurt their feelings.” If so, he surely ought to have written about his late son in what is after all an autobiography? But no. “I felt I had nothing interesting to say about him,” he says, sighing. “Anything I said would have been as cliched as a 1930s B movie.”

You leave thinking that he would have made a perfect spy.