A man who last week became the first person in Britain to be cleared of deliberately infecting someone with the HIV virus could be the first of many - according to a leading HIV charity.

Matthew Collins was acquitted of grevious bodily harm in what has been described as a landmark ruling at Kingston Crown Court on Wednesday after Judge Binning directed the jury to return a not guilty verdict.

Central to the case was testimony from Dr Anna-Maria Geretti, a consultant clinical virologist at the Royal Free Hospital in London, who said that scientific tests were not able to conclusively prove that an infection was passed from one particular person to another.

In this case, the infected person was having sex with more than one person who was HIV positive, meaning the defendant was not the only possible source of infection.

Lisa Power, head of policy at the Terrence Higgins Trust said: "Other people facing similar charges have been advised to plead guilty because of this kind of evidence. I think this case will have an impact on further cases of this kind.

"Medical evidence can show that two people have the same virus, but the science is so complex it is very difficult to prove transmission from one person to another."

Recent high profile cases of HIV infection include father-of-three Mohammed Dica, from Mitcham, who in October 2003 became the first person in 137 years to be convicted for sexually transmitting a disease in England and Wales.

In June this year Sarah Jane Porter, from Kennington, was jailed for 32 months for infecting a sexual partner with HIV.

In previous cases, virological similarities in two people infected with the virus have helped to prove transmission between them.

But Dr Geretti said such evidence alone should not lead to a conviction.

She told the Comet: "Virological evidence should be seen in the context of other facts. You should not build a case around this type of evidence alone. There could be a chain of transmission, where four or five people are infected with a similar virus, so it is impossible to tell whether transmission has occurred between two people with a related virus."

Paul Hayward, press officer for the Criminal Prosecution Service said future cases with similar evidence will have to be considered carefully.

"If cases have similar evidence to the reasons for this aquittal we would make sure that the evidence was robust enough for a reasonable prospect of conviction."