Health service executives urged clients of Iketam Clinical Laboratory Services in Balham, south London, to seek help immediately after Sir Kenneth Calman, the Government's Chief Medical Officer, wrote to GPs warning that the laboratory's diagnoses may be incorrect.
The health alert centres round inquiries which have established that Nigerian-born Godwin Onubogu, 56, who was practising at the laboratory, is not a registered medical practitioner.
In a joint statement, the South Thames NHS Executive and the Merton, Sutton and Wandsworth Health Authority, alleged yesterday that Mr Onubogu ran Iketam as a private concern for nine years, offering testing and advice on "various conditions, including sexually transmitted diseases and HIV infection".
In his alert to doctors, Dr Calman said: "Some patients may need counselling. Others may need referral to sexually transmitted disease-genito urinary medicine clinics as they may have been inaccurately diagnosed as having gonorrhoea.
"Others may have had HIV tests, the results of which may not be reliable, and may need referral to HIV services." He said those affected may not be limited to the Balham area but could hail from other parts of London, the South-east or even abroad. Dr Sue Atkinson, director of public health at the executive, said: "We are concerned about the information that some people may have been given by Mr Onubogu and so we are trying to contact his clients and former clients to alert them to the need to seek professional medical advice. The best route for this would be through their GP.
"Iketam is not an NHS laboratory, so we do not have details of the clients. The police ... have the names and addresses of some clients. We have given the police letters to enclose with the ones they are sending to these people. This still leaves many clients who we cannot contact and who may not live in the local area."
The NHS executive said only people who actually visited Iketam's premises at Swan House, 207 Balham High Road, needed to seek further help as the laboratory did not conduct diagnoses on samples sent from other doctors.
"We don't want to cause a panic among people who have had HIV tests and may worry that their tests were sent off to this laboratory - they weren't," said Kate Wilcox, a spokeswoman for the NHS executive.
"We only want to contact people who visited the premises. Obviously, this is going to be an anxious time for some people, but we would urge them to contact their GP as soon as possible."
The health authorities have set up a helpline - on 0800 174846 - for anyone who has received treatment, testing or diagnosis at the laboratory.