The letters apologise for any distress caused and say: "Although I would like to say again that the risk of any patient contracting HIV from this dentist is small, we do feel that everyone has the right to know about even this small risk and to be counselled about any action they should consider taking."
The dentist, Vikram Advani, had an NHS practice up to 1991 when he was diagnosed but he continued in private practice until June 1993. He died in November of the same year.
The 1,285 patients involved lived or live in Kensington & Chelsea and Westminster and in Camden and Islington health authority areas. Because of the time lapse, officials have had to trace patients through 60 local health districts in England.
In the letters, each patient has been given a personal telephone number to one of four centres set up to offer advice, counselling and HIV tests. An additional freephone line, 0800 374 135, has been set up for any other patient who is anxious.
At the time of Mr Advani's death there was media publicity, and about 1,100 people telephoned a helpline. Many callers were anonymous. There will be some overlap in the patients who telephoned in 1993 and those who received letters yesterday.
John James, chief executive of the Kensington health authority, said at a briefing yesterday: "To date there is no recorded case anywhere in the UK of the virus having been transmitted to a patient from a health worker. Nonetheless, the authorities believe it is right that every patient should be informed about the risk however small."
Mr James said there had been consultation with the UK Advisory Panel for Healthcare Workers Infected With Bloodborne Diseases which suggested the "look-back'' exercise. He said it was a "very difficult decision" and he was aware the letters would cause "anxiety and distress". It was felt, however, that the authorities "owed it" to patients to warn them.
"At this stage we do not know whether any patient will be found to be HIV positive as a result of this exercise, but any who are will be offered professional advice and counselling."
Worldwide, 22,000 patients have been followed up after being treated by 63 HIV positive health care workers, including 33 dentists. But no cases of HIV transmission have come to light.
One well publicised case of a Florida dentist who died in 1991 continues to be clouded by controversy. Six people claimed they had been infected but there has never been conclusive proof.
The risk of transmission is low and needs blood to blood or body fluid contact in a sufficient dose to transfer infection. Even then, there are many variable factors including the immune state of the recipient and the level of infectivity of the "donor"
John Hunt, chief executive of the British Dental Association, called for a compensation scheme to help health workers admit they are infected. "I am certainly not condoning any dentist practising while he or she has Aids," he said. "On the other hand, you have to consider what we can do to stop a person who is determined to contravene all ethical and moral obligations ... We should be looking at some form of compensation scheme so they can come forward and acknowledge they are positive and withdraw from practice."