Police chief says plans laid out in modern slavery bill for legal discretion will help bring cases against gangs exploiting people

Granting victims of slavery immunity from prosecution to give evidence about exploitation will be a key tool in tackling organised gangs, according to the officer in charge of coordinating new investigative powers.

The modern slavery bill, due to pass into law before the election, will help the 13,000 people estimated to be working under duress in the UK but may also put pressure on police resources, warned Shaun Sawyer, chief constable of Devon and Cornwall.

Sawyer, who is lead officer on slavery for the Association of Chief Police Officers (Acpo), said people are being forced to work in increasingly diverse circumstances: in car washes, cannabis farms, brothels, meat packing factories, on fishing boats, in nail bars, domestic servitude and as pickpockets.

His comments were made as the College of Policing puts out to consultation on Wednesday guidance to help frontline officers identify and investigate modern slavery. He told the Guardian that the most important approach for officers is to “see the victim first, see the crime second”.

The modern slavery bill contains a provision giving police and prosecutors wide discretion in whether to ignore minor offences committed by victims of slavery in order to assemble cases against those who exploit them.

“It’s the only way from my perspective to get the best evidence and to bring serious and organised criminals to book,” Sawyer explained. “If you are forced to commit a crime but you are enslaved it would seem to be against natural justice to [be] prosecuted.” Anyone who committed a serious offence, such as murder, would not escape prosecution, however.

Officers should keep an open mind when dealing with the vulnerable who have been enslaved and not assume they behaved like others, Sawyer added. “I don’t think unless you have been enslaved you can get your mind around the level of disempowerment,” he said. “Officers might say: ‘Surely you had access to a car or a phone [and could escape easily]’ if someone had been in domestic servitude.

“It’s about keeping professionally curious. Don’t assume. We might say surely they could take control but they are mentally if not physically disempowered from taking any action.”

Between 10,000 and 13,000 people are believed to be effectively working as slaves in the UK, according to Home Office research. Many victims are workers from abroad who are not aware of their rights and are being exploited, others are vulnerable individuals from the UK who have been threatened or bullied into subservience.

“There has been an explosion of demands on the police,” Sawyer said. “Traditional crime has fallen away but it’s being replaced by complex work which is far more demanding. And how far do you go back in time? At the moment we have the resources. Will we have enough for the future? This will be a lot of work. It will continue to change the nature of policing.”

The College of Policing’s draft guidance emphasises that investigations should be “victim-focused”, with the primary objective to safeguard and support victims.

It states that “victims are often very vulnerable but may not appear to be so”. Training on “how to identify and apply the indicators is essential to spot a potential victim”. There is no definitive victim profile, it adds, it can happen to anyone.

“Early financial investigation in tandem with criminal investigation is very important in order to access the organised crime groups behind the slavery offences and to recover assets,” it says.

The modern slavery bill introduces a statutory defence for victims of modern slavery who have been compelled to commit an offence; the requirement for compulsion will not apply to children.

The defence will not apply to a number of serious offences – mainly violent and sexual offences – set out in the bill. It will cover the offences that victims of modern slavery are typically forced to commit such as cannabis cultivation, offences related to prostitution and immigration.