Of the 99 suspects, 46 had their citizenship bids turned down but likely remained in Britain, 20 were refused asylum and the fate of the remaining number was unknown, the BBC reported Tuesday.
Human rights advocates have called for the government to conduct more criminal investigations into suspected war criminals who have not been deported to their home countries. British courts commonly block the deportation of people who may be subjected to torture or the death penalty in their native countries.
"There's a cost and it is difficult to investigate crimes which took place in another country a long time ago. But if we don't pursue those prosecutions, the U.K. could become known as a retirement home for war criminals," said James Smith of the charity Aegis, which works to prevent crimes against humanity.
The Metropolitan Police said 56 people in Britain are being investigated as war criminals, though the Home Office reports only nine cases, the BBC said.
MP Michael McCann said that disparity indicates immigration officials may be wrongly identifying people as war crimes suspects.