A scandal involving a member of the House of Lords is exposed today – but this one occurred 34 years ago and the only harm caused was to anyone lacking a sense of humour.

Baroness Elaine Murphy, a cross-bench peer and former Professor of Psychiatry at Guy's Hospital, has confessed to manufacturing a medical condition which only existed in her imagination.

In a letter to the British Medical Journal published in 1974, the young Dr Murphy claimed to have discovered "cello scrotum", a painful affliction which only affected male players of the instrument.

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The spoof letter was written in response to an earlier one about "guitar nipple" from a Dr P Curtis, which the young Dr Murphy thought likely also to be a spoof. To disguise her identity she persuaded her former husband, John Murphy, now chairman of St Peter's Brewery, Suffolk, to sign the letter, although he was a theoretical chemist, not a medical doctor.

Their secret was kept for more than three decades until a researcher writing in the 2008 Christmas issue of the BMJ cited cello scrotum among the health problems of musicians in an article entitled "A symphony of maladies".

In a letter to the BMJ published today, the Baroness and her co-conspirator said the citation of their "discovery" in the Christmas BMJ prompted them to confess to their youthful prank.

"Reading Curtis' letter on guitar nipple, we thought it highly likely to be a spoof and decided to go one further by pretending to have noticed a similar phenomenon in cellists. Anyone who has ever watched a cello being played would realise the physical impossibility of our claim," they write.

Despite this, and to their astonishment, the original letter was published by the presumably musically illiterate then letters editor of the BMJ. The mystery deepened when, the following Christmas, the Murphys sent a card to Dr Curtis addressed to "The inventor of guitar nipple from the discoverers of cello scrotum."

It turned out that Dr Curtis knew nothing about the condition he had supposedly identified – "another joke, we suspect", they write.

Their letter continues: "We have been dining out on this ever since. We were thrilled once more to be quoted in "A symphony of maladies". The BMJ has dubbed the episode "Scrotumgate" in a tribute to the political scandal that had engulfed Washington DC the previous year.

Baroness Murphy had a distinguished 25-year career as a doctor and academic in the NHS before moving into NHS management. She has been chair of the North East London Strategic Health Authority and is chair of council at St George's, University of London.

She was made a life peer in 2004 and speaks mainly on mental health, education, healthcare and ageing issues. She has remarried and lives in Norfolk.

Fiona Godlee, editor of the BMJ, said: "It seems the BMJ has been deliciously hoaxed. It is wonderful it has been going all these years and no one realised. We frown on misconduct and medical fraud is taken very seriously. But in this case I hope I am right in saying that no harm has been done." The letter is illustrated by a cartoon of a naked cellist in pain – caused by his instrument, or by being the butt of a 34-year-old jape.