Moves to shut down websites used to sell sex have been abandoned by the National Crime Agency.

Under a new strategy, the agency will instead advise the “adult services” websites on how to “improve their standards” to stop the exploitation of slaves.

Those running the websites will be told how to verify the age of the women or men selling their bodies and to be alert for suspicious signs such as several people being advertised by one person.

Other warning indicators include “bait-and-switch” adverts, where a photograph of a woman is used to lure a customer who finds a different person when they arrive to have sex.

Law enforcers say this can indicate that one slavery victim has been swapped for another — or that the woman selling sex is doing so because she is being controlled by organised criminals.

The NCA’s aim is to persuade those who run the websites to stop them being used to promote unlawful activity.

The approach is likely to be controversial as it amounts to a tacit endorsement of prostitution — which some campaigners view as inherently exploitative — by Britain’s leading law enforcement body.

But Adam Thompson, head of the NCA’s modern slavery and human trafficking unit, insisted that pragmatic methods were likely to be more effective.

“The marketplace for sexual services is not on the street any more — it’s on the internet and traffickers take advantage of that and advertise victims through these adult services websites,” he said.

“We’ve seen other countries like the US taking down some of these websites, but our assessment is that risks simply displacing the problem to other parts of the internet.

“You get into this ‘whack a mole’ situation where you take down one website and a new one pops up.

“What we want to do is encourage these websites to improve their standards and to work with us — a bit like the banks have introduced ways of identifying suspicious activity — to improve the safety of people who are being advertised on these sites.”

He added: “It’s not illegal to buy sex in the UK at the moment so what we want them to do is to improve their standards. There are some basic things those websites need to be doing — looking on a regular basis through all adverts going on their sites and looking to see what might indicate suspicion.

“Multiple different people being advertised by a single person, signs of coercion and control which websites might be able to see from who’s paying — and then for websites to share that information with law enforcement.

“Some websites are more willing than others to engage with us but the pace of change is too slow. Tech companies need to be more proactive, invest more of their resources into safeguarding activity, identifying potential victims and making referrals to police.”

Figures for the Government’s National Referral Mechanism, which records those identified as suspected slaves, show that nearly 2,000 victims of forced sexual exploitation were found last year, including more than 630 children.

Law enforcers say that many victims suffer repeated sexual abuse and violence.