Essex police have refused to confirm they will not share data with the Home Office on the immigration status of anyone responding to recent public appeals for information on the deaths of 39 people in a lorry.

The force last week urged those who may have information to come forward “without fear”.

Making a direct appeal to anyone living illegally in the UK who may be able to help with the investigation, it said any information would be received in the strictest confidence “and no criminal action will be taken against you”.

However, when asked to confirm the immigration status of those responding would not be shared with the Home Office, Essex police refused to comment.

It is currently police policy to share data with the Home Office on the immigration statuses of victims of crime if information gathered by the police indicates they might be living illegally in the UK.

The refusal to give specific assurances raised concerns among legal professionals and anti-trafficking experts that those coming forward may not understand that this could lead to their data being shared with immigration officials.

“The police need to confirm that they will not be data-sharing nor reporting witnesses or victims to the Home Office,” said Ahmed Aydeed from Duncan Lewis Solicitors, who claimed the police had become an extension of the Border Force.

“The police also need to confirm that when they conduct intelligence checks during their investigation, and it is apparent that the relevant person is suspected of being an illegal entrant, they will not contact immigration enforcement. Unless the police provide cast-iron guarantees on this, it is unlikely relevant people will come forward,” he said.

Although the Modern Slavery Act 2015 established a legal framework of protection for those in modern slavery in the UK, trafficking victims are still routinely criminalised for immigration offences. Research last year found more than 500 trafficking victims were sent into immigration detention instead of being referred to support services.

Parosha Chandran, a human rights barrister specialising in modern slavery, said she was concerned the appeals for information would undermine efforts to build bridges between authorities and communities affected by human trafficking in the UK.

“There needs to be a joined-up approach by the police and the Home Office to understand that when trying to investigate crimes of human trafficking, they are often going to need to rely in part on communities that are not traditionally known for having confidence in their treatment by the authorities,” she said.

“The police need to be very careful that they are not betraying the trust of frightened and vulnerable people, who wish to come forward but who may have been trafficked or exploited here themselves.”

The risk that going to authorities would lead to victims being arrested, detained and deported is often used by traffickers as a means of coercion and control. Kate Roberts, the UK and Europe programme manager at Anti-Slavery International, said the deaths of the 39 people believed to have been smuggled or trafficked into the country should prompt a “system change” in how human trafficking offences were investigated.

“The call for a firewall between immigration enforcement and policing crimes targeted at the very people made vulnerable by their lack of status has been made time and time again,” she said.

“These deaths which have horrified us all need to be a catalyst for system change. That means guarantees that people who come forward with information on a crime will not have their information passed onto the Home Office.”