We're trying to remove the stigma against sex work and sex workers.

Ana Lopes, International Union of Sex Workers

They come from all over the world and many face exploitation or violence.

The International Union of Sex Workers, based in London, has been campaigning to improve conditions.

The union's secretary, Ana Lopes, says joining one of Britain's biggest unions will strengthen its voice on issues such as the decriminalisation of prostitution in the UK.

"We're trying to remove the stigma against sex work and sex workers," Ms Lopes said.

"We think that changing the law - decriminalising sex work - is one of the steps towards it and it's a very important one.

Legal change

"The same happened with gay rights, when they removed the laws that actually helped social attitudes to change as well."

Britain's laws on prostitution are not clear cut. Selling sex is legal but soliciting for trade is not.

Ms Lopes says the current laws increase the risk of violence, forcing prostitutes to rely on pimps for protection.

The GMB says it will welcome the sex workers as new members, but they will have to lobby within the union before it presses for a change in the law on prostitution.

'Breakthrough'

If, as expected, the sex workers do vote to join, it will help them campaign for better pay and conditions and it will offer training and advice on other careers.

Ms Lopes says this will be an important breakthrough for sex workers beyond Britain.

The only other European country where they have mainstream union support is the Netherlands.

She acknowledges that some GMB members may be unhappy about campaigning for better rights and conditions alongside sex workers, but she hopes that these concerns will subside over time.