There could be between 10,000 and 13,000 victims of slavery in the UK, higher than previous figures, analysis for the Home Office suggests, BBC reported.
Modern slavery victims are said to include women forced into prostitution, "imprisoned" domestic staff and workers in fields, factories and fishing boats.
The figure for 2013 is the first time the government has made an official estimate of the scale of the problem.
The Home Office has launched a strategy to help tackle slavery.
It said the victims included people trafficked from more than 100 countries - the most prevalent being Albania, Nigeria, Vietnam and Romania - as well as British-born adults and children.
Data from the National Crime Agency's Human Trafficking Centre last year put the number of slavery victims in the UK at 2,744.
The assessment was collated from sources including police, the UK Border Force, charities and the Gangmasters Licensing Authority.
The Home Office said it used established statistical methodology and models from other public policy contexts to estimate a "dark figure" that may not have come to the NCA's attention.
It said the "tentative conclusions" of its analysis is that the number of victims is higher than thought.
The Modern Slavery Bill going through Parliament aims to 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, co-ordinated 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.
Modern slavery minister Karen Bradley told the BBC she was not surprised by the figures.
She said: "This is very much a hidden crime and the important thing is that we get it out in the open. If we compare where we were 200 years ago, the anti-slavery campaigners there had to make people acknowledge that slavery was wrong.
"What we have to do today is not make people acknowledge it's wrong - everybody knows it's wrong - but we have to find it.
"It's a hidden crime, it's going on in streets, in towns, in villages across Britain and we need to help people find the signs of it so we can find those victims and importantly then find the perpetrators."
Aidan McQuade, director of charity Anti-Slavery International, said the Home Office's figures "sounded about right" but questioned whether the government's strategy went far enough.
He told the BBC : "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."
Aneeta Prem, founder of the Freedom Charity, said recent publicity around the issue was helping, but "everyone needs to be vigilant."
She said: "It could be someone forced into sex trafficking, someone forced to work on a farm with no pay or little pay."
She added: "It could be somebody that's working in a car wash, somebody that you just suspect is in the wrong situation. We can all spot these signs and hopefully report it and get something done about it."