The UK says it is unwilling to confirm that a former Sri Lankan rebel got into the UK after the Sri Lankan government gave him a passport with a false name.
Col Karuna used to be one of the Tigers' top commanders
Colonel Karuna Amman, once the second in command in the Tamil Tigers, is in detention in the UK.
He was given the diplomatic passport in August, a newspaper in Sri Lanka has now revealed.
Human rights groups have called on the UK government to prosecute him for human rights abuses.
Colonel Karuna is one of the most controversial figures in Sri Lanka's protracted civil war.
He split from the Tamil Tigers in 2004, forming a breakaway faction.
He is later said to have collaborated with government forces and helped them to take control of territory in the east of the country.
Earlier this year, the New York-based Human Rights Watch published a report which accused Col Karuna's group of abducting and forcibly recruiting at least 200 Tamil children in Sri Lanka's eastern districts.
An investigation by Sri Lanka's Morning Leader newspaper shows that the fake passport for Colonel Karuna was issued under the name Kokila Gunawardena.
Child recruitment has been a feature of Sri Lanka's civil war
He was then given a UK visa by the British High Commission in the Sri Lankan capital after a recommendation by the Sri Lankan foreign ministry, the paper says.
The UK Home Office has confirmed that Colonel Karuna is now "being held in immigration detention".
A spokeswoman told the BBC news website that the Home Office "is unwilling to confirm" that he was given a UK visa under false pretences.
The Morning Leader also said that Colonel Karuna had applied for asylum in the UK.
The Home Office has not ruled out returning him to Sri Lanka.
"The 1951 Refugee Convention allows us to exclude persons from the protection of asylum where there are serious reasons for considering that an individual has committed a serious crime or other acts which make him or her undeserving of international protection," it told the BBC.
"The convention also provides that in some circumstances persons can be returned to their country of origin, even though they may have a well-founded fear of persecution there."
However, it said it would not comment on individual cases.
"We don't comment on individual cases, however where evidence comes to light of involvement in war crimes, cases will be investigated."