Up to 13,000 people in Britain are being held in conditions of slavery, four times the number previously thought, the Home Office has said.
The number of victims last year was between 10,000 and 13,000 according to new data, said to be the first scientific estimate of the scale of modern slavery in the UK.
The figure includes women forced into prostitution, domestic staff and workers in fields, factories and fishing boats.
Data from the National Crime Agency’s Human Trafficking Centre previously put the number at 2013 at 2,744.
Launching the government's modern slavery strategy, home secretary Theresa May said the scale of abuse was "shocking".
“The first step to eradicating the scourge of modern slavery is acknowledging and confronting its existence,” she said.
“The estimated scale of the problem in modern Britain is shocking and these new figures starkly reinforce the case for urgent action.”
Dark figure
The new estimate is based on a statistical analysis by the Home Office chief scientific adviser, Prof Bernard Silverman, which aims for the first time to calculate the "dark figure" of victim numbers 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 new strategy document, which builds on the frameworks used to counter terrorism and fight organised crime, sets out plans for co-ordinated action across government and law enforcement agencies to run alongside the Modern Slavery Bill currently going through Parliament.
In her foreword to the document, Mrs May said: “Young girls are raped, beaten, passed from abuser to abuser and sexually exploited for profit. Vulnerable men are tricked into long hours of hard labour before being locked away in cold sheds or run-down caravans.
“Women are forced into prostitution, and children systematically exploited. Domestic workers are imprisoned and made to work all hours of the day and night for little or no pay.”
While many victims are foreign nationals, the document says that vulnerable British adults and children are also being systematically preyed upon by traffickers and slave drivers.
The National Crime Agency estimates that the UK was the third most common country of origin for victims identified in 2013.
Among overseas victims, many of them brought into the UK by people traffickers, the most common countries were Romania, Poland, Albania and Nigeria.
PA