LONDON (AP) _ Three medical researchers say a British sailor who died in 1959 had the AIDS virus, which would mean the disease was present in Britain more than 20 years earlier than had been thought.

The researchers at the medical school at the University of Manchester reported their findings in The Lancet, a London-based weekly medical journal.

The report, made available today, was based on tests on tissue preserved from the Royal Navy sailor's body.

According to the report, the sailor had an immune deficiency that at the time could not be explained, as well as pneumocystis pneumonia, one of the opportunistic diseases now associated with AIDS.

The discovery stemmed from the persistence of one of the three researchers, pathologist George Williams, who performed the original examination of the sailor's body.

In a report in The Lancet in the 1960s, Williams described it as a unique case of pneumonia accompanied by overwhelming complications and secondary infections.

When AIDS was recognized in the 1980s, Williams reminded his colleagues of the similarity and asked if they could reopen the sailor's case. By then, the sailor's only physical remains were samples of tissue stored in paraffin blocks.

In their report in The Lancet, Williams, virologist Andrew Bailey and Gerald Corbitt reported that the HIV virus was found in tissue taken from the sailor's kidneys, bone marrow, spleen and pharyngeal mucosa.

HIV, or human immunodeficiency virus, causes acquired immune deficiency syndrome, or AIDS.

''Taking into account the incubation time of several years between infection and development of the full-blown disease, the seaman would have contracted the virus in the early 1950s,'' the Times of London quoted Bailey as saying in an interview.

The researchers said the earliest accepted case of human blood serum with antibodies indicating the donor was HIV-positive was a sample taken in Zaire in 1959.

AIDS breaks down the body's immune system, leaving victims susceptible to a wide variety of infections and cancers. It is usually transmitted through sexual contact, blood transfusions or infected needles. There is no known cure.

As of June 1, 1990, the World Health Organization counted a cumulative total of 263,051 cases of AIDS worldwide, but WHO estimates the true number to be closer to 700,000.

The organization says 156 countries have officially reported at least one case of AIDS.