Kits allowing people to test themselves for HIV at home can be bought over the counter in the UK for the first time – but no kits exist yet in Britain.
The change in the law means it is now legal for people to test and diagnose themselves at home.
Previously, people could carry out tests they ordered online at home and send away their results, but were diagnosed over the phone.
It is hoped the move will help the UK’s 25,000 undiagnosed HIV-positive people.
No tests have yet been developed that pass European guidelines, said the government’s health regulator.
Home testing for HIV was approved by the government last September but the law only came into effect on Sunday.
The Terrence Higgins Trust HIV charity said the tests could be introduced into the UK this year or in early 2015.
The UK is leading Europe in making the kits available over the counter, but they were introduced to the US in 2012.
Testing could involve taking a small drop of blood from a finger, or a swab from the inside of the mouth.