About 10,613 potential victims were referred to the British government for help last year - three fewer than in 2019 - according to the Home Office (interior ministry), which said the plateau was mainly caused by Covid-19 and lockdown restrictions.
A rising number of children were identified in 2020 - 47 percent of all referrals up from 43 percent- fueled by a rise in reports of "county lines" exploitation, with drug gangs using young people to move their wares from cities into rural areas.
Anti-slavery charities said the pandemic meant frontline professionals - from police to social workers - had fewer opportunities to identify and engage with possible victims, while some resources were diverted into the Covid-19 response.
"Sadly, this decrease in referrals is not due to there being less victims ... on our streets. In fact, quite the opposite," said Robyn Phillips, the London Project and Survivor Lead for the Human Trafficking Foundation.
"The pandemic has exacerbated gaps in intelligence about victims who were being exploited by businesses that are closed; and with ... a decline of care services and police operations, opportunities for identification have decreased," she added.
The Home Office was not immediately available to comment on the statistics. The ministry has previously said it is committed to tackling the crime and ensuring that victims receive support.
People who say they have been enslaved enter the National Referral Mechanism (NRM) and access care, from housing and healthcare to legal aid, while the government considers their claim. The process can take from weeks to several years.
About three-quarters of the suspected victims referred to the NRM last year were male, the data showed, and British, Albanian and Vietnamese were the most common nationalities.
Alex Balch, director of research at the Modern Slavery Policy and Research Center, said the increase in referrals for suspected child victims of the county lines trade - which rose by about a third to 1,371 in 2020 - was "very worrying."
"It is good that awareness of this form of exploitation is rising, but we need to know more about it in order to improve the protection of children," Balch said in a statement.
Activists have told the Thomson Reuters Foundation the NRM figure likely represents only the "tip of the iceberg" and that many foreign victims decide not to enter the system because it does not allow them to work or guarantee asylum if successful.
A study last year by charity Justice and Care and The Center for Social Justice think-tank said Britain was home to at least 100,000 slaves - 10 times more than a 2013 official estimate.
Despite being hailed as a global leader in the anti-slavery drive, Britain's landmark 2015 law has faced widespread criticism that it is not being used fully to jail traffickers, drive companies to tackle forced labor, or help enough victims.