(Reuters) Men with same-sex sexual partners are 28 times more likely to contract HIV than their heterosexual counterparts despite a radical reduction in new infections among gay men in Western countries, a U.N. report said on Wednesday.

The overall annual number of new HIV infections dropped from a high of 3.4 million in 1996 to 1.8m last year, according to UNAIDS, which co-ordinates the organisation's global response to HIV/Aids.

Yet gay men and men who have sex with men remained most at risk of contracting HIV, along with female sex workers, drug users and transgender women, the Global Aids Update 2018 report noted.

In North America, western Europe and Australia, the roll-out of pre-exposure prophylaxis (PrEP) - an oral prophylactic - has seen transmission rates drop dramatically among gay men.

Take-up of PrEP, sold commercially as Truvada, saw new infections fall by 43 percent in San Francisco in three years. The Australian state of New South Wales reported a similar drop of 35 percent over two years since its introduction.

In Britain, however, PrEP remains controversial.

Last year, following a court battle with one of Britain's leading HIV/Aids charities, the National Health Service embarked on a three-year trial of Truvada among 10,000 gay and bisexual men and men who have sex with men.

Charities and campaigners on Tuesday called for the trial numbers to be expanded.

"We are aware of one person who was turned away and subsequently became HIV positive," said Matthew Hodson, executive director of NAM, a British HIV/Aids information charity. Read more via NDTV