People at high risk of HIV are being offered a test that can deliver results in 20 minutes.
The move by Barts and The London NHS Trust is the first time that a mouth swab HIV test has been made available on the NHS.
A trial has shown that the saliva test is as effective as a blood test and doctors are expecting to carry out around 250 screenings per month.
Of those surveyed, three in four said they preferred it to the conventional test - which takes a week before results are available.
The new test uses a special absorbent toothbrush to collect saliva samples from the mouth, and is currently being offered to anyone from areas of high HIV prevalence, gay and bisexual men, and other people at increased risk of infection.
Merie Symonds, head of sexual health at the trust, said: "As this test doesn't require us to take blood or send samples to a laboratory, we can begin to look towards testing outside traditional clinical areas.
"We are already offering this test to service users in some of our outreach services run in non-healthcare settings."