LONDON (AFP) - Up to 13,000 people are estimated to be kept in conditions of slavery in Britain, four times higher than was previously thought, officials said Saturday.
The Home Office figure for 2013 includes women forced into prostitution and people forced to work in factories and fields, many of them foreign nationals.
Data from another government body previously put the figure for 2013 at 2,744, but the 13,000 number is an estimate that factors in the "dark figure" of undetected cases, it said.
The figure emerged as Prime Minister David Cameron's government launched a strategy to combat modern slavery, with new legislation likely to be passed next year.
"Be in no doubt, slavery is taking place here in the UK," Home Secretary Theresa 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 rundown caravans," she said.
"People are made to work in fields, in factories, and on fishing vessels. Women are forced into prostitution, and children systematically exploited."
May added: "We must send a powerful message to all traffickers and slave drivers that they will not get away with their crimes."
The Home Office defines modern slavery as including "slavery, servitude, forced and compulsory labour and human trafficking."
The estimate of up to 13,000 was based on a statistical analysis of existing data by the Home Office's chief scientific adviser, Professor Bernard Silverman.
He said the figure should be treated as "tentative" because his analysis dealt with assumptions which, while "plausible", are not easily verified.