More potential slavery victims were referred to police for investigation in London last year, new figures show.
Anti-trafficking charity Hope for Justice said many police forces lack specialist offers to deal with these cases, meaning not enough perpetrators are being investigated and prosecuted.
Figures from the Home Office show the Met Police was responsible for investigating 5,435 referrals where a person was a potential victim of modern slavery, including 1,947 children, in the year to June.
Overall, it was up from 4,601 in 2021-22.
Adam Hewitt, a Hope for Justice spokesperson, said the identification of victims and referral is an important step towards survivors getting support.
He added: "Unfortunately, even after a referral, many survivors face incredibly long delays in having a final decision made on their case – often years.
"A lack of resources and specialised teams at many police forces, plus the complexity of many cases, mean not enough perpetrators are being investigated and prosecuted."
Across the UK, more than 17,700 potential victims of modern slavery were referred to the Home Office in 2022-23 – a significant jump from 14,600 the year before.
Of the referrals last year, nearly 7,600 were children.
Despite the annual increase, the most recent quarterly data for April to June saw the lowest level of referrals since the start of 2022.
Andrew Wallis, chief executive of anti-slavery charity Unseen, said this is a result of the Government's move to change Home Office guidance earlier this year requiring claimants to prove they are victims by submitting third-party evidence of modern slavery and trafficking.
Legal challenges by two people who were refused protection under the new threshold led to the Government conceding and withdrawing the new guidance last month.
Mr Wallis said: "To the scale of the problem, everybody accepts – except maybe the Government – that at any one time in the UK somewhere in excess of 120,00 victims of modern slavery are in a situation of exploitation.
"And, even though we have found nearly 18,000, sadly most of those victims will be replaced by another person."
He added: "Until every police force, officer, statutory agency, and society as a whole understand what modern slavery looks like, we will never be able to say that we have a true picture of the size and scope of modern slavery.
"But the more we look for it, the more we find it."
A Home Office spokesperson said the Government remains commited to stamping out modern slavery.
They added: “We are reforming our response to modern slavery to make the system more robust and ensure that genuine victims are supported.
"This includes new rules introduced in January this year, meaning that individuals who pose a threat to public order are able to be disqualified from using modern slavery protections to thwart their removal from the UK. These measures have also been expanded through the landmark Illegal Migration Act.”
Anyone who has any concerns, or intelligence to suggest modern slavery or human trafficking is happening in their communities, can contact the Modern Slavery Helpline on 08000 121 700.