Vinayagmoorthy Muralitharan, also known as Colonel Karuna, was released Thursday following his imprisonment for possessing false documents, the British High Commission (embassy) said here in a statement.
"We have made clear our concerns at the circumstances by which Karuna travelled to the UK.... He (Muralitharan) now remains under immigration detention powers," the statement said.
Colonel Karuna had been sentenced to nine months jail for entering Britain illegally. He was reportedly arrested in London in November.
Karuna had travelled to Britain on an apparently genuine Sri Lankan diplomatic passport issued under a false name. It is not clear if he will be deported.
But human rights group Amnesty International has called for British authorities to investigate whether he can be prosecuted in London for international war crimes, including abducting and using child soldiers.
But that appeared unlikely.
"The Crown Prosecution Service has advised the Metropolitan Police Service