The number of slavery victims helped in England and Wales has risen almost fivefold since 2012, according to the Salvation Army.
The charity said more than 1,800 people were given support between April 2015 and March 2016 - up from 378 between July 2011 and June 2012.
In the past five years, the group has helped nearly 4,500 modern slaves, but Home Office figures estimate there are still between 10,000 and 13,000 slaves in the UK.
Almost half had been sexually exploited, 42% had been subject to forced labour and 13% had been held in domestic servitude, the charity said. Sixty-two percent were women and 38% were male, with six transgender people supported.
The highest number of referrals were Albanian, with large numbers from Poland, Nigeria and Vietnam, as well as a "significant proportion" of British citizens. Almost one third of victims were trafficked to London.
Since its first year of running the government "victim care" contract in 2011, the Salvation Army has reported a year-on-year rise in the number of modern slaves being identified.
Anne Read, director of anti-trafficking and modern slavery for the Salvation Army, said: "Perpetrators go to great lengths to hide their crimes so it is always difficult to know the full extent of the problem. It may be that the increases in the numbers of people being supported is because there are more victims.
"The greater the awareness of this issue the more difficult it will become for traffickers to ply their evil trade in human beings."
Prime Minister Theresa May introduced the Modern Slavery Act last year while she was home secretary to put "slave masters behind bars where they belong", calling it "the great human rights issue of our time".
She said 289 modern slavery offences were prosecuted in 2015 along with a 40% rise in victims identified by the Government.
Mrs May added that more than £33 million would be used to focus on "high-risk countries", where people were regularly trafficked to the UK and forced into servitude.
Referred victims have a 45-day "reflection and recovery period" to allow them to "recover from their ordeal and reflect on what they want to do next", according to the National Crime Agency.
Victims from nearly 100 countries were being supported, the charity said.