Home Secretary Theresa May has promised a crackdown on modern slavery in Britain in the wake of the discovery of three women who had been held captive for up to 30 years in a south London property.

May told an international women's rights conference in London that the government was drawing up the Modern Slavery Bill which would bring in tougher sentences and create a a position of an anti-slavery commissioner.

"The steps we're taking will help this country reach the point where we never ignore this evil, never allow slave masters and those who look to exploit other women, men and children to think the UK is a safe place for them to operate in," she told the Trust Women conference.

A man and a woman believed to have been part of a Maoist sect in south London, were arrested in November and accused of keeping a 69-year-old Malaysian, a 57-year-old from Ireland and the 30-year-old British woman captive for decades.

May said that human trafficking, sexual exploitation, domestic servitude, forced marriage and labour, were all the faces of modern slavery.

"We all know that there are countless more examples of this hidden crime at this very second, in this very country," May said. "This is simply unacceptable in modern-day Britain."

Monique Villa, head of the Thomson Reuters Foundation charity, which organised the conference with the International New York Times, said the extent of modern slavery was such that most people had probably unknowingly met a victim.

"Most of us here today have probably met a modern-day slave without knowing it. It can be on a bus, in a nail salon, in a posh hotel. Anywhere - in London, in New York, in Dubai. They walk among us," Villa said.

"One can think that slavery belongs to the history books, but far from it. It's worse today than ever."

According to the foundation there are almost 30 million slaves around the world, including more than 4,500, in the UK.