In principle: a clinic cannot test you without your consent. Any test or treatment done without your consent or knowledge is unethical and an assault upon you.

So in the majority of cases it doesn't happen. But it does no harm at all to be quite explicit and tell the doctor that you do not want to be tested for HIV antibodies.

If you're worried because you automatically had blood taken at an STI clinic, this is because they routinely do a blood test for syphilis, and not every doctor will explain this. So in itself this is not a sinister sign of testing for HIV antibodies without your consent.

However, it would be very hard to check if a particular doctor had tested you without your consent. If you have good grounds to think this might have happened, make an appointment with the consultant and carefully explain your concern.

If this interview does not satisfy you, you should get help in making a complaint. Legal advice can be obtained from Terrence Higgins Trust.