She is known to generations of children as the saintly, iron-willed Lady With the Lamp who battled to improve the conditions of wounded British soldiers and founded modern nursing, but a strikingly different picture of Florence Nightingale has emerged from the unpublished letters of one of her bitterest enemies.

"Miss Nightingale shows an ambitious struggling after power inimical to the true interests of the medical department," Sir John Hall, the chief British army medical officer in the Crimea, wrote to his superior in London.

When she went over his head to order supplies from his stores, observers, Sir John wrote, were astounded at the "petticoat imperium! in the medical imperio!"

When Nightingale arrived in Scutari in November 1854 with 38 women volunteers, sent by her close friend, the war secretary, Sydney Herbert, she was about to carve out her place in history and destroy Sir John's. Her determination to reform the army hospitals in which thousands of wounded and ill soldiers were treated in closely packed beds by overworked doctors and male medical orderlies, and untrained women whom she dismissed as drunken and slatternly, brought her into instant collision with Sir John - and she also became a media star in the first British war reported in detail by the press.

"It was absolutely as night follows day that her upper-class Victorian female morality would clash head on with his traditional closed male army world," said Richard Aspin, head of the archive and manuscripts at the Wellcome Trust, which recently bought Sir John's letters. "She simply ignored his authority. She would no more have dreamed of consulting him about her nurses than she would have sought the opinion of a husband, if she ever had one, about hiring a parlour maid."

Sir John's letters denounced her as a publicity seeking meddler. Her ambitions, which launched the modern career of nursing, "if not resisted", he wrote, "will, with the influence she has at present at home, throw us completely into the shade in future, as we are at present overlooked in all that is good and beneficial regarding our hospital arrangements, which are ascribed utterly to her presiding genius by great part of the press and her own itinerant eulogistic orators".

He accused her of squandering resources by sacking good nurses and orderlies and trying to take over control of others - "but in that she was disappointed, for they declined to serve under her orders".

It might be some consolation to poor Sir John that the scruffy marbled notebook containing his transcripts of the letters he considered most important cost the Wellcome Trust £4,000, while Nightingale's letters were bought for only £200. One letter from Nightingale, advising on how to find a reliable medical officer for a post in Egypt, warns against employing ex-army doctors: "The fact is, nearly all the half-pay list are blackguards".

Henry Wellcome, who founded the trust, shared the general reverence for the Lady with the Lamp. Hers was the only woman's name he included in the frieze of his library, and he bought the scuffed mocassins she wore at Scutari - now on view in the new museum galleries which opened in London this summer. The collection also owns, but has lent to the British Library, the only known recording of Nightingale's voice, on a wax cylinder.

Hall battled on, writing in February 1856: "The army is in splendid health, only seven deaths in a week and one of them a fit of apoplexy from drunkeness."

However, his view of history's treatment of Nightingale and himself was prophetic. He wrote sadly: "We shall to the end of time be made the victims of public odium in the way we were last winter ... the poor suffering sick soldier is a fine theme to ride off on."

When his long military service was rewarded with a knighthood, Nightingale commented to Sydney Herbert that the honour could only mean "knight of the Crimean burial grounds".

A disputed legacy: What the historians say

Most biographies of Florence Nightingale attest that she became a national hero after dramatically reducing the mortality rate at the Scutari hospital during the Crimean war. But new research casts doubt on her role in transforming the hospital after her arrival in 1854. Official records show that by February 1855, the mortality rate had fallen from 60% to 42.7% and then, once a fresh water supply was introduced, it dropped further to 2.2%.

Recently historians have suggested the death rate among soldiers did not fall immediately but rose, and was higher than any other hospital in the region. During Nightingale's first winter there, 4,077 soldiers died, mostly of typhus, typhoid, cholera and dysentery. Ten times more died of these illnesses than from battle wounds.

The death rate began to fall six months after she took charge - only after a sanitary commission was sent out by Lord Palmerston to improve ventilation and clean out the sewers.

Nightingale had believed the mortality rates were due to poor nutrition and overworking of soldiers. But Hugh Small, author of Florence Nightingale: Avenging Angel, claims an unpublished letter shows it was not until 1857 that she realised the conditions within the hospitals themselves had caused such a huge number of deaths.

Lee Glendinning