May called for a greater urgency in tackling a borderless crime affecting 46 million people worldwide and generating $150 billion in illegal profits a year.

"To the victims of modern slavery: We will not ignore your plight," she said, speaking at London's Westminster Abbey. "We will not turn away. We will not shut our eyes and pretend your suffering does not exist.

"We will work tirelessly, relentlessly pursuing the perpetrators of these appalling crimes so that victims of slavery can go free. And my message to these criminals is simply this: We are coming after you."

Britain last year passed tough anti-slavery legislation introducing life sentences for traffickers and forcing companies to disclose what they are doing to make sure their supply chains are free from slavery.

Last month, May pledged to use 33.5 million pounds ($42 million) from the foreign aid budget to focus on combating slavery in countries which victims are known to be trafficked to Britain, where an estimated 11,700 people are enslaved.

"This is a global phenomenon that knows no geographical boundaries, crossing not just borders but over the internet," she said.

"So we need a radical domestic and international approach to target every aspect of this despicable trade and strip the slave drivers of the profit they make out of human suffering by putting them behind bars."