After 58 years, at the age of 95, Alistair Cooke signs his very last 'Letter From America'

You thought it would never happen; but yesterday, to the sorrow of millions of listeners across five continents, it did. Alistair Cooke yesterday announced his retirement, and after 58 years the BBC's Letter from America, the world's longest-running speech radio programme, is no more.

In a sense, the end was predictable, after the 95-year-old Mr Cooke was forced by illness to skip last Friday's talk. On the advice of his doctors, he then decided to give up for good. "I have had much enjoyment doing these talks," he said, and added what must be a contender for understatement of the year. "I hope that some of it has passed over to the listeners."

Indeed it did. No one so succeeded in explaining to the English-speaking world the complexities of America behind the screaming headlines, and the idiosyncrasies of a country at once so familiar to Britain, and yet so utterly foreign.

As a radio broadcaster, Mr Cooke had three unrivalled strengths. One was his ability to approach a subject sideways, evident in his penultimate Letter from America on 13 February, dealing with weight problems. It dealt with slimming, but gave only a glancing mention to the present fuss here about obesity and the low-carb Atkins' diet.

Instead, Mr Cooke produced a vintage Cooke historical riff, on how slimming in 1915 had become a necessity amid the deprivations of the First World War, and how a Florida boffin during the Second World War found a way of concentrating orange juice to send to vitamin-C-deprived British children on the other side of the Atlantic, and a parenthetical aside on some late 19th-century quack's potion that was supposed to cure lung cancer.

Mr Cooke's second great asset was his style at the microphone. He spoke in a rich and weathered mid-Atlantic accent. His wrote his scripts not as an essay to be read on the printed page, but in the syncopated, somewhat random way people actually talk to each other.

The final ingredient was his sheer professional longevity. He had seen almost everything and could write about anything, from the dreaded "yips" at golf to a moving eyewitness account of the assassination of Robert Kennedy in a Los Angeles hotel in June 1968 .

The technique injected a suspense into his every broadcast: where would he veer off next? More than any other programme, Letter from America illustrated how radio can be so infinitely more satisfying a medium than television.

Its origins in March 1946, were humble. "Why don't you talk about the things you talk to me about," his producer suggested. "American children, the chemistry of the New England fall, out West, anything?" The initial run would be 13 weeks, with a further 13 if the programme was a success.

But that would be it: the Treasury had virtually banned the export of sterling. "So even if you're the biggest thing that ever happened at the end of 26 weeks ... no more." But as Mr Cooke drily remarked more than half a century later, "Somehow, they forgot."

But Letter from America was only one facet of almost 60 years of reporting on the country. For two decades he was the Manchester Guardian correspondent in America. In the 1950s, he presented Omnibus, a cultural show which broke new ground on US television.

Later, he presented Masterpiece Theatre on public television, a cult programme of classy British series. Then came the TV documentary, Alistair Cooke's America, a smash on both sides of the Atlantic which generated a best-selling book and an invitation for Mr Cooke to address Congress in its bicentennial celebrations in 1974.

A year earlier, the man who had become a US citizen in 1941 was granted an honorary knighthood by the Queen. But it was the congressional appearance that best symbolised how far he had come since his birth in 1908 as plain Alfred Cooke in Salford, near Manchester, and his early years in a Blackpool boarding house.

But he made it to Cambridge, took a degree in English, changed his name to Alistair and joined the BBC in 1934 as a film critic. Shortly after he made his first visit to America, and decided telling the world about it would be his mission in life. So he became a national monument in Britain and his adopted America. But his attitude was a quintessential piece of English understatement.

Today, Mr Cooke is largely confined to his 15th-floor apartment in New York, with its fabulous views across Central Park. "I do not think I have been any help at all," he told Nick Clarke, his biographer, in 1999. "I just think it's a great privilege for anyone who knows both countries well to be able to watch two different kinds of human beings."