Boris Johnson's reported plans to create an immigration and visa system separate from the Home Office has experts fearing that the reach of the hostile environment will be expanded.

The prime minister is said to be formulating proposals to create a new department for borders and immigration to improve security and the operation of the visa system after Brexit, along with a number of other “big changes” to the structure of government, according to the Sunday Times.

There is little detail on how this move would take effect, but lawyers and campaigners cautioned that while it had the potential to present an opportunity to make the system "fairer", the focus was more likely to be on tightening control of the borders.

Experts also warned that if not implemented with adequate time and resource, such a move could cause considerable upheaval and have potentially negative consequences on the ability to deliver immigration and visa services effectively.

Tanja Bueltmann, an academic specialising in migration, speculated that judging by the “recent history” of the Conservative party’s approach to immigration, the new department would likely place an intensified focus on “limiting numbers”.

“It will expand the structural reach of the hostile environment. We’re setting up another government department to look after, by the sounds of it, another aspect of immigration management. This would give the hostile environment even more oomph," she said.

“We have to wait and see, but if you look at the US for example, you’ve got the Department of Homeland Security and then the Immigration and Customs Enforcement agency (Ice). I’d be paying attention to whether this is going to turn into something like that."

Immigration solicitor Jonathan Kingham said that while the best case scenario would be for the new department to “recognise the unfairness of the system” and try to create a more “fair” decision-making process, the focus appeared to be on control and reducing migration.

“They’re emphasising security and so forth. The mood music is very much towards restriction and control,” he added.

Omar Khan, director of the Runnymede Trust, said taking immigration responsibility out of the Home Office would in theory be a good thing, but that under Mr Johnson’s leadership a new department was unlikely to learn lessons from previous mistakes such as the Windrush scandal.

“The culture of a department is driven partly by its aims and objects, and driving numbers down is quite clearly going to be the focus of this immigration department. They’re not going to be to look holistically and the human beings in front of them,” he said.

“That was the major problem with Windrush. People were ignoring the merits of the case. But they haven’t even learnt from the mistakes from the past on all of that. We had Mr Johnson himself basically saying that migrants don’t belong here last week.

“We’ve said before that it’s time to shut the Home Office down. But as long as the attitude is sceptical, and as long as it’s about driving numbers down, the institutional machinery doesn’t really matter.”

Experts also raised concern that transferring immigration responsibilities away from the Home Office and to a brand new department would risk causing disruption and upheaval in the system.

Joe Owens, Brexit programme director at the Institute for Government, said that creating a separate department for borders and immigration had the potential to help rebuild public trust in the immigration system, but that if not carried out effectively it could be damaging.

He added: “The Home Office has had a difficult few years in terms of public confidence and trust in the immigration system, be it Windrush or EU citizens and the settlement scheme. The lack of trust in this government institution is sometimes quite startling.

“So there are a few reasons why it’s worth looking at, but it definitely won’t mean the problems will just disappear.

Why is the Home Office getting so many immigration decisions wrong?

“The best case scenario is this is a well-thought through, planned move which addresses some of the structural issues in the way the Home Office works at the moment. The worst case is that they drag and drop the exact same system and the same structures into a new department, and rather than fixing anything they just disrupt all of the work that’s going on there.

“So the risk is you get poorer decisions, troubles with the EU settlement scheme and the delivery of that, and you get issues with the building of the new immigration system. Adding that level of disruption in could have some negative consequences to their ability to actually deliver.”