As many as 13,000 people in Britain are being held in conditions of slavery, four times the number previously thought, it has been revealed.
In what is said to be the first scientific estimate of the scale of modern slavery in the UK, the Home Office has said the number of victims last year was between 10,000 and 13,000.
They include women forced into prostitution, domestic staff and workers in fields, factories and fishing boats.
Data from the National Crime Agency's Human Trafficking Centre had previously put the number of slavery victims in 2013 at 2,744.
Theresa May, the Home Secretary, said the scale of abuse was "shocking".
Launching the Government's modern slavery strategy, she said: "The first step to eradicating the scourge of modern slavery is acknowledging and confronting its existence. The estimated scale of the problem in modern Britain is shocking and these new figures starkly reinforce the case for urgent action."
The Modern Slavery Bill, currently going through Parliament, aims provide courts in England and Wales with new powers to protect people who are trafficked into the countries and held against their will. Scotland and Northern Ireland are planning similar measures.
But outlining the strategy for government departments, its agencies and partners, Home Secretary Theresa May said legislation was "only part of the answer".
The "grim reality" is that slavery still exists in towns, cities and the countryside across the world, including the UK, she said.
"The time has come for concerted, coordinated action. Working with a wide-range of partners, we must step up the fight against modern slavery in this country, and internationally, to put an end to the misery suffered by innocent people around the world."
The Home Office said the UK Border Force would roll out specialist trafficking teams at major ports and airports to spot potential victims, and the legal framework would be strengthened for confiscating the proceeds of crime.
Aidan McQuade, director of charity Anti-Slavery International, questioned whether the government's strategy went far enough.
He said: "If you leave an employment relationship, even if you're suffering from any sort of exploitation up to and including forced labour, even if you're suffering from all sorts of physical and sexual violence, you'll be deported.
"So that gives an enormous power in the hands of unscrupulous employers. And frankly the protections which the government has put in place are not worth the paper they're written on in order to prevent this sort of exploitation once they've given employers that sort of power."
The new estimate is based on a statistical analysis by the Home Office chief scientific adviser, Professor Bernard Silverman, which aims for the first time to calculate the "dark figure" of victim numbers who are not reported to the law enforcement agencies.
"Modern slavery is very often deeply hidden and so it is a great challenge to assess its scale," he said.
"The data collected is inevitably incomplete and, in addition, has to be very carefully handled because of its sensitivity. "
The Telegraph UK.