But it was indeed too good to be true. It now seems Wilson was the victim of an elaborate hoax. The poet, who was born 100 years ago today, never penned the note.

The telltale sign that the letter is a joke is that the capital letters at the start of each sentence spell out "A N Wilson is a shit". A journalist drew the biographer's attention to the coded message last week, and after rereading the letter he admitted that it was a hoax. "Of course I saw the funny side - I laughed about it a lot when I found out," Wilson told the Guardian yesterday. "It is quite childish of somebody and I have absolutely no interest in who wrote it."

The biographer, who estimates that he read about 50,000 Betjeman-related letters while researching his book, said there was little he could have done to avoid being caught out. "If I went through every single letter to check the start of each sentence I wouldn't have written the book."

Wilson said the discovery had "come at a good time" because the book was about to be reprinted anyway. He estimated that around 13,000 copies had been printed originally, but another 30,000 are to be printed without the letter. He speculated that the few copies containing the hoax may be sought by book collectors.

Details of the hoax come just days after the Guardian revealed there were tensions between Wilson and Bevis Hillier, who wrote an authorised Betjeman biography. Hillier, who worked for 25 years on his three-volume biography, suggested Wilson may have used him as his "unpaid research worker" by drawing on his material. But Wilson, who described Hillier's second volume as a "hopeless mishmash of a book" in a review for the Spectator in 2002, denied using any of Hillier's work for research purposes. Hillier has denied any involvement in the hoax.

The letter, which surfaced two years ago while Wilson was researching the biography, was addressed to Honor Tracy, an Anglo-Irish writer with whom the poet worked at the Admiralty during the war. It appeared to have been written by the poet in May 1944 - 11 years after he married Penelope Chetwode.

When it came to Wilson's attention it was accompanied by a note in which"Eve de Harben" wrote that she had received the letter from her father, a cousin of Tracy. Tracy died in 1989. De Harben, who wrote from an address in France, sent what was claimed to be a typed copy of the love letter. The trickster said the original had been sold to an American collector of memorabilia related to the poet. Wilson has said he did think it odd, when he finally returned the letter to De Harben at the stated address, that it was returned with "addressee and address not known".

Betjeman became poet laureate in 1972, and died in May 1984.

The letter

Darling Honor,

I loved yesterday. All day, I've thought of nothing else. No other love I've had means so much. Was it just an aberration on your part, or will you meet me at Mrs Holmes's again - say on Saturday? I won't be able to sleep until I have your answer.

Love has given me a miss for so long, and now this miracle has happened. Sex is a part of it, of course, but I have a Romaunt of the Rose feeling about it too. On Saturday we could have lunch at Fortt's, then go back to Mrs H's. Never mind if you can't make it then. I am free on Sunday too or Sunday week. Signal me tomorrow as to whether and when you can come.

Anthony Powell has written to me, and mentions you admiringly. Some of his comments about the Army are v funny. He's somebody I'd like to know better when the war is over. I find his letters funnier than his books. Tinkerty-tonk, my darling. I pray I'll hear from you tomorrow. If I don't I'll visit your office in a fake beard.

All love, JB