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.
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 (NCA) 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.
Concerted action call
The Home Office said it had 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... 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.
The modern slavery strategy will also see:
- The government identify "priority countries" to work with, as well as other organisations including churches
- British embassies and high commissions and NCA liaison officers develop local initiatives abroad
- Work to strengthen the response by local authorities to child abuse, including trafficking
- Work to raise awareness among homeless shelter staff of the signs of modern slavery
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.
Allan Doherty, of the charity Hope for Justice, said traffickers used a number of ways to control their victims.
"They will maybe threaten them in some cases with physical violence, or threaten their families back home and, of course, they take documents off victims and it makes it incredibly difficult for that victim to get away and to go and find any help.
"There are many cases where the victim does try and get help from the authorities and lots of opportunities are missed because the authorities don't understand the crime, and don't recognise them as victims."
Aneeta Prem, founder of the Freedom Charity, said recent publicity around the issue was helping, but "everyone needs to be vigilant."
She said: "It's not somebody in shackles, it's not somebody tied into a house that cannot leave, that isn't what a modern day slave is.
"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."