The Government needs to improve its understanding of modern day slavery if it is to effectively counter the crime, a watchdog has warned.
Home Office data offers an incomplete picture of the perpetrators and victims, the National Audit Office (NAO) said, adding that oversight of victim support is inadequate with few cases leading to a prosecution.
"The campaign to drive out modern slavery is in the early stages," Amyas Morse, head of the NAO said.
"So far it is helping to establish the scale and international nature of this issue. To combat modern slavery successfully, however, government will need to build much stronger information and understanding of perpetrators and victims than it has now."
Modern slavery encompasses a range of criminality including servitude, forced labour and human trafficking.
An official estimate previously suggested there are up to 13,000 potential victims in the UK, although the anti-slavery commissioner has described the figure as "far too modest".
As part of its inquiry the NAO reviewed the National Referral Mechanism, a framework established in 2009 to identify victims of human trafficking or modern slavery and ensure they receive appropriate support.
The watchdog's assessment said referrals to the NRM increased substantially between 2014 and 2016, meaning more potential victims are being identified.
But the report said the Home Office has "limited means" of tracking its progress and "there remains much more to do to ensure victims of modern slavery are identified, protected and supported effectively".
Analysis of the NRM's data revealed "multiple errors" and duplicate entries - making it difficult to use the data to understand the crime, according to the NAO.
It also flagged up the length of time authorities are taking to decide whether people referred to the mechanism are victims of modern slavery.
This is "causing further distress and anxiety to the vulnerable people in the system", the report said.
For two thirds of those referred in 2016-17, the Government took longer than 90 days to make a "conclusive grounds" decision.
A Home Office spokeswoman said: "Modern slavery is a barbaric crime that destroys the lives of victims across the globe.
"We welcome the National Audit Office report's recognition of the work we have done to identify the issue and put in place the ambitious Modern Slavery Strategy and the Modern Slavery Act 2015 - the first legislation of its kind in the world.
"Since the National Audit Office examined our work on modern slavery we have made significant strides in a number of areas they identify in their report.
"Recently announced reforms to the National Referral Mechanism will make substantial improvements to a system which is supporting more victims than ever before."