The first legally approved HIV self-test kit that allows people to get a result in 15 minutes at home has gone on sale in England, Scotland and Wales.
Unlike other kits, these tests do not need to be sent off to a lab to get the results.
It works by detecting antibodies on a small drop of blood, which are often only detectable three months after the infection is caught.
Experts warn that any positive tests must be reconfirmed at clinics.
Charities hope it will reduce some of the 26,000 people estimated to have undiagnosed HIV in the UK.
An early diagnosis allows people to get treatment quickly and can prevent serious complications. And individuals successfully treated for HIV are less likely to pass the infection on.
This new "do-it-yourself" test is made by company Bio Sure UK and can be bought online.
It works in a similar way to a pregnancy test, measuring levels of antibodies - proteins made in response to the virus - in a person's blood.
The device analyses a small droplet of blood, taken from the finger-tip using a lancet. Two purple lines appear if it is positive.
The company recommends attending sexual health clinics for advice and further blood tests if both lines appear.
And even if the test is negative experts say it does not mean people are definitely virus free - especially if exposure occurred within the last three months.
The three-month window period, between the moment someone catches the infection and the time it can take for antibodies to develop, means the kit is not reliable during this time.
Charities have welcomed the test and hope it will encourage more people to get checks - particularly those reluctant to go to clinics in the first instance.
Dr Rosemary Gillespie, chief executive at Terrence Higgins Trust, said: "We campaigned for a long time to secure the legalisation of HIV self-test kits which happened in April 2014, so it is great to see the first self-test kits being approved.
"However, it is important to make sure people can get quick access to support when they get their result."
Shaun Griffin, also at the charity, said: "At the moment there are funding challenges throughout the NHS, including for sexual health services.
"It is absolutely critical that people have access to HIV tests and advice they need."
Free HIV tests are available across the NHS.
In Northern Ireland ministers are considering legal changes to allow the sale of home testing kits.