Some 49 people suspected of committing genocide, torture or other serious crimes abroad are apparently living in Britain despite being slated for deportation.
The individuals are said to have been given "restricted leave" to stay in the UK after judges ruled they could be harmed in their home countries.
Details obtained by BBC Radio 4's Today programme under freedom of information rules indicate they come from places including Iraq, Zimbabwe, Sri Lanka and Eritrea.
Another recent case involved a former Taliban fighter.
Christopher Cole, chairman of the Law Society's immigration law committee, said denial of refugee status combined with human rights rules on deportation could result in a "sort of legal limbo".
"Unless there is a fundamental change in many of these (people's) country of origin, then realistically there isn't any prospect of many of these people being removed.
"Certainly not within six months, and unlikely within six, 10, 12 years - who knows?"
All 49 cases of restricted leave involve Article 1F of the Refugee Convention, which prevents individuals being given asylum in the UK if they have been involved in war crimes, terrorism or other serious crimes abroad.
But the European Convention on Human Rights means they cannot be legally removed from Britain to their home country because of the risk of torture or even death.
Charges can be brought in the UK for the worst crimes, such as genocide, but police often struggle to assemble evidence.
A Home Office spokeswoman said restricted leave was kept under constant review, and individuals could have conditions put on employment, residency and education.
Would-be immigrants are screened for possible involvement in serious offences, and investigations will take place into allegations of war crimes.
"The Government is determined that the UK should not become a refuge for war criminals and those who have committed crimes against humanity," the spokeswoman said.
"Anyone accused of these crimes should be put on trial in their home country and we will always seek to return them to face justice."