The number of people held as modern-day slaves in the UK is four times worse than feared, a new Home Office study has found.

It is the first time the government has made an official estimate of the number of slavery victims in the country.

They include women forced into prostitution, imprisoned domestic staff, and field and factory workers.

It is estimated that last year between 10,000 and 13,000 people were trapped in conditions of slavery.

Previous figures for the same year, released by the National Crime Agency's Human Trafficking Centre, put the number at 2,744.

The new estimate is designed to take into account the "dark figure" of victim numbers which go undetected by authorities, the Home Office said.

"Modern slavery is very often deeply hidden and so it is a great challenge to assess its scale," said Home Office chief scientific adviser Professor Bernard Silverman, who compiled the statistical analysis behind the figures.

According to the National Crime Agency, a large proportion of the victims documented in 2013 were from overseas, although the UK was the third most common country of origin.

Other common countries of origin included Romania, Poland, Albania and Nigeria.

Home Secretary Theresa May has said urgent action is now needed to confront the "shocking" levels of abuse uncovered.

"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."

The British government has published a wide-ranging strategy aimed at fighting modern slavery. 

It sets out plans for coordinated action across departments, agencies and law enforcement in the UK and internationally.

The Home Office said the strategy will build on the frameworks used to counter terrorism and fight organised crime in order to pursue those responsible, try and prevent additional cases and raise awareness for the victims.

The strategy will run alongside new legislative powers to be granted to courts as part of the Modern Slavery Bill currently going through Parliament.