MPs have expressed surprise over the appointment of one of Britain’s most senior police officers as Theresa May’s anti-slavery commissioner, eight months after the previous postholder resigned citing government interference.

Sara Thornton, chair of the National Police Chiefs’ Council, will take up the post later this year, the Sunday Times reported, although the Home Office did not confirm the appointment. A candidate would be “announced shortly” it said.

Thornton, a former chief constable for Thames Valley police, is seen as a controversial appointment. She was forced to apologise for the force’s failings in a child sex slavery scandal in Oxford in 2013, when hundreds of girls who were later found to have been groomed and sexually abused were not initially believed by Thames Valley officers.

Frank Field, MP, one of three politicians who reviewed the Modern Slavery Act and expressed concern over the independence of the commissioner’s post, said the appointment was “so surprising” that he would be seeking to instigate parliamentary scrutiny of the role, post-appointment.

Field, Lady Butler-Sloss and Maria Miller, MP, who published an interim review in December, urged the government to give up its search for a new anti-slavery chief and address concerns about the independence of the role before re-advertising. They had recommended the appointment should be subject to parliamentary scrutiny, at pre-appointment hearings, but that had not happened, Field said.

“It is so surprising, this appointment, that I will be seeking agreement of Yvette Cooper that it carries out such scrutiny,” Field said.

Cooper, the chair of the home affairs select committee, is undertaking a review of policy related to modern slavery. The Home Office is currently reviewing the Modern Slavery Act.

Field voiced concern that the appointment was once again a police officer, which emphasised the enforcement nature of the role.

Campaigners for slavery victims echoed his fears that the Thornton had a law enforcement background.

Bharti Patel, the chief executive officer of Ecpat, Every Child Protected Against Trafficking, said: “We are concerned we have another police officer. We have concerns that the position needs to be victim focused and does not just have a law enforcement aspect. We need a clear and adequate support system for children.

“In his review of the Modern Slavery Act, Frank Field was very keen that the position should be strong and robust, given that the current system has failed. This appointment is absolutely vital for us … We hope that Sara will push for a legal system of guardianship for vulnerable children.”

Hyland, appointed as the inaugural independent anti-slavery commissioner in 2014, as part of Britain’s Modern Slavery Act, resigned in May last year, citing a lack of independence in the role.

Aidan McQuade, slavery expert and former director of Anti-Slavery International, said: “Given the background of the candidate, I would have said it looks like a controversial appointment, but sometimes people who have made mistakes in the past can use their experience to become transformative in the future. On that basis, I would wish her the very best of luck.”

McQuade has made no secret of his unsuccessful application for the post of commissioner. In an opinion piece in January, entitled “A poisoned chalice? Challenges for the UK’s next anti-slavery commissioner”, published by Thomson Reuters, McQuade said the new commissioner would be forced to grapple with government policies that have detrimental effects on those affected by slavery in the UK. These included the “ferocious hostile environment” policy that gave rise to the Windrush scandal and the 2016 immigration act, which created the criminal offence of “illegal working”, an “additional direct threat” to the protection of slavery victims, he said.

The Home Office has advertised twice for the role – a three-year contract with a salary of up to £140,000.

Kate Roberts, head of office at Human Trafficking Foundation, said: “If the appointment is confirmed, we would hope that anyone in post would learn from their own past experiences of the need to support vulnerable people. The person in post should have the independence to step back from other agendas such as immigration control.

“We would like to see this move away from a criminal justice perspective into one of prevention, to look at what makes people vulnerable to traffickers and to look at hostile environment.”

Field’s interim report identified a catalogue of constraints on the Commissioners independence and concluded: “We are concerned by the statements of several stakeholders that the Commissioner was not free to scrutinise and criticise government policy and performance in addressing modern slavery.”

Hyland said that the Home Office should not be in charge of decisions relating to slavery victims.

“Home Office staff do not have the skills or the experience to make decisions about victims of serious and organised crime, which is what modern slavery is,” he said.

In the face of mounting criticism of its treatment of modern slavery victims, in October 2017, the Home Office pledged to radically improve support for victims of trafficking and slavery, and said it was putting the welfare of victims and potential victims at the heart of its anti-slavery agenda.

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

A Home Office spokesperson said: “The Independent Anti-Slavery Commissioner (IASC) plays a crucial role in providing scrutiny of the government’s response to modern slavery.

“We are currently undertaking the recruitment process to appoint a candidate, which will be announced shortly.”