Survivors of modern slavery in Britain should no longer have support cut off six weeks after they are officially recognised as victims by the government, the High Court ruled on Wednesday.

The decision - an interim measure as part of a ongoing legal review into Britain's system for identifying and looking after slaves - may pressure the Home Office (interior ministry) to boost support for victims, lawyers and campaigners said.

"In my judgment, there is a real risk of irreparable harm to a significant number of vulnerable victims of slavery and trafficking if their support were to end after 45 days," High Court judge Julian Knowles said in his ruling.

The Home Office said it would not be appropriate to comment on ongoing legal proceedings.

"The government is committed to ensuring all victims of modern slavery get the support they need," a spokesman said.

Despite being hailed as a global leader in the anti-slavery drive, Britain is undergoing a review of its landmark 2015 law amid criticism that it is not being used fully to jail traffickers, drive firms to stop forced labour or help victims.

People who say they have been enslaved can enter the National Referral Mechanism (NRM) and access care - ranging from healthcare and housing to legal aid - while the British government decides whether or not to recognise them as victims.

But this support ends 45 days after someone is formally identified as a victim, a policy that two survivors - an Albanian victim of sex trafficking and a Vietnamese man forced to work in a cannabis factory - are challenging in court.

In the first hearing of the case last month, a judge said that the existing rules were potentially unlawful.

"We owe them an obligation to provide support on the basis of need, rather than ... how long it has been since they were conclusively identified as victims," said Harvey Slade, a caseworker at Duncan Lewis that is representing the two victims.

"It is vital that all victims of trafficking and slavery continue receiving support and assistance on the basis of need while the lawfulness of this policy is fully scrutinised."

With at least 7,000 suspected slavery victims uncovered in Britain last year - up a third on 2017 - according to data published last month, activists have raised concerns about the government's ability to support a growing number of survivors.

The Home Office in 2017 announced several reforms to the NRM, including extra shelter and support, drop-in services and an overhaul of the decision-making and review processes.

Yet activists say the short period of support leaves victims at risk of homelessness, destitution or being exploited afresh.

"The court ruling is essentially saying that the current system is unsafe," Tamara Barnett, project leader at the Human Trafficking Foundation, told the Thomson Reuters Foundation.

"The process needs to be more victim-focused, not so blunt and short-term, and stop putting people at risk," she said, adding that the ruling could spur Britain to consider a proposed bill that would guarantee victims support for at least a year.

Britain is home to at least 136,000 modern slaves, according to the Global Slavery Index by rights group Walk Free Foundation - a figure 10 times higher than a government estimate from 2013.