UPDATE 1-UK will not charge ex-S.Lanka Tiger with war crimes

(adds Human Rights Watch comment paragraphs 2, 14-15)

By Peter Apps

LONDON, May 9 (Reuters) - Britain has released former Sri Lankan Tamil Tiger rebel Karuna Amman from prison where he was detained on immigration charges, it said on Friday, believing he could not be convicted of war crimes.

Amnesty International and Human Rights Watch expressed disappointment, saying police should continue to investigate the former eastern rebel commander. They accuse him of abuses including torture and child soldier recruitment both before and after his 2004 split from the mainstream rebels.

The British High Commission in Colombo and Home Office (interior ministry) in London said Karuna, whose real name is V. Muralitharan, remained in immigration detention in Britain.

Rights groups say Karuna could be deported to Sri Lanka, apply for asylum in Britain or simply linger for months or perhaps longer in detention while his fate is decided.

But Britain says he is unlikely to face charges despite a range of allegations from rights groups.

"We are deeply concerned that Karuna and his faction had allegedly been responsible for murder and abductions and are still believed to be involved in intimidation and child recruitment," the British High Commission in Colombo said in a statement.

"The Crown Prosecution Service has advised the Metropolitan Police Service that there is insufficient evidence to provide a realistic prospect for conviction of any criminal offences in the UK."