Escaping modern slavery
"Whilst Britain has made progress fighting modern slavery, not least in the passing of the Modern Slavery Act five years ago, so much more work is needed to combat the issue," the charity's chief executive, Christian Guy, said in a statement.
The world-first law has been subjected to reviews following criticism that it is not being used fully to jail traffickers, drive companies to tackle forced labour, or help enough victims.
"Ninety percent of victims may be going undetected and thousands of traffickers are running riot," Guy added.
A record 10 627 suspected victims were identified last year in Britain - up 52% from 2018. Most were victims of labour abuse and many came from nations such as Albania, Nigeria and Vietnam.
Britain's home secretary Priti Patel said her department would look closely at the report's findings.
"While the Modern Slavery Act was ground-breaking in tackling this heinous crime, we are just getting started in the fight to rid this evil from the United Kingdom," Patel said.
Government research in 2018 said the crime sets Britain back by up to 4.3 billion pounds ($5.6 billion) annually, based on an earlier estimate of 10 000 - 13 000 slaves living in the nation.
The charity and the think tank said the government needed to update its estimate, and the cost of the crime to the taxpayer.
The study arrived at its 100 000 estimate by taking police statistics from one area of the UK - the West Midlands - that predicted the number of victims locally using a new artificial intelligence model, and extrapolating that data nationwide.
Academics at Nottingham University's Rights Lab, the world's first large-scale research platform on slavery, said the study offered a "very exciting and new way" to measure the problem.