The UK is discriminating against Kenyans with HIV by insisting that people who want visas be screened for tuberculosis, Aids activists say.
It is illegal to discriminate against people with HIV
Inviolata Mbwavi of the Empowerment Network for People Living with Aids says those with HIV will be most affected by the directive.
TB is one of the diseases which often infects people with HIV, because of their weak immune systems.
The British embassy in Kenya says it is trying to halt the spread of TB.
It says the directive is already being implemented in Tanzania and Sudan.
From 5 February, visa applicants will be refused unless they have a certificate to show they do not have TB.
Medical experts say about 100,000 people in Kenya get tuberculosis annually.
"Tuberculosis is common among HIV-positive people and this means that they will not able to travel to Britain," Ms Mbwavi told the BBC.
"To us they are tactfully discriminating us and it's against international law."
More than 15,000 Kenyans, mainly students, apply for visas to travel to the UK each year.
Ms Mbwavi said Aids activists in Kenya will now launch a global campaign to push the UK to withdraw the directive.