Theresa May has criticsed Rishi Sunak over his plan to water down the modern slavery laws she introduced.
The former prime minister said his proposed changes - which would make it harder for alleged victims to remain in the UK - could be exploited by criminal gangs.
Sunak announced two weeks ago that he wanted to “remove the gold plating in our modern slavery system”.
That would force those claiming to be modern slaves to provide more evidence to prove they are not making bogus allegations.
The prime minister told MPs: “One of the reasons we struggle to remove people is because they unfairly exploit our modern slavery system.
“So we will significantly raise the threshold someone has to meet to be considered a modern slave.
“For the first time, we will actually require a case worker to have objective evidence of modern slavery rather than just a suspicion.”
Sunak made the announcement as part of moves to crack down on illegal immigration.
But in an interview with Radio 4′s PM programme broadcast today, May - who oversaw the passing of the Modern Slavery Act when she was in Number 10 - said his plan could “create another potential loophole”.
She said: “For example, there’s talk of requiring more evidence from individuals.
“If you’re somebody who’s been trafficked here as a sex slave, and you manage to find your way out of that and look to somebody for help, the chances are you probably haven’t got a piece of paper or a written statement from somebody to say ‘you’ve been in slavery’. The evidence comes gradually.
“If you are somebody who is being brought by a criminal gang who are abusing the system, and they know there needs to be a piece of paper, they probably will provide a piece of paper. So it’s making sure that, in dealing with problems that are identified we don’t create other problems.”
Elsewhere in the interview, May said Sunak had made good progress in restoring the Tories’ reputation following Liz Truss’s disastrous time as PM.
And she insisted her party could still win the next election, despite Labour’s lead in the opinion polls.
She said: “There’s no doubt that the mini-budget had an impact on the Conservative Party’s reputation for sound money and sound public finances.
“But I think what we’ve seen already with the new chancellor and a new prime minister is taking the process of re-establishing that reputation.
“And I think from everything we’ve seen from Rishi so far that actually, he’s going to be able to turn that round by the next election. I see that we can in – those two years – show people that a Conservative government can be on their side and that he can turn it round and we can win that election.”
