Between 10,000 and 13,000 people in the UK are victims of slavery.
The analysis is the first time the Home Office has put out an official estimate of the scale of the situation.
British-born adults and children are in the number as well as victims trafficked from more than 100 countries, particularly Albania, Nigeria, Vietnam and Romania.
The victims include women forced into prostitution, domestic staff allowed no liberty, and people working on farms, in factories and on fishing vessels. Mostly are forced to work against their will with little or no pay.
The Home Office put together its figures, higher than previously thought, from sources such as police, border control, charities and the Gangmasters Licensing Authority and has reached a “tentative conclusion”.
A Modern Slavery Bill is going through Parliament and a Modern Slavery Minister, Karen Bradley, has been appointed.
Home Secretary Theresa May said the fight against slavery must be stepped up with coordinated action across government departments, agencies and partners.
Specialist teams at ports and airports will attempt to spot potential victims, the government will work with key countries and embassies will develop initiatives abroad. At home, local authorities will have to strengthen their response to child abuse, including trafficking – not before time.
