A prominent human rights watchdog has criticised the British government for allowing Vinayagamoorthi Muralitharan alias Colonel Karuna Amman, a former Tamil Tigers leader, to return to Sri Lanka [Images] as a free man.

45-year-old Karuna was released from a jail in London [Images] last month after serving three months for entering Britain on a forged visa and diplomatic passport.

In a statement, Human Rights Watch (HRW) described him as "abusive former Tamil Tiger leader".

It also urged the Sri Lankan government to investigate and prosecute him for alleged war crimes committed by him as commander of the Liberation Tigers of Tamil Eelam (LTTE).

Karuna was the top commander of the LTTE in eastern Sri Lanka, and the reputed number two in the organisation until he left to form his own armed group in March 2004.

Because his group fought against LTTE in recent years, HRW said, the Sri Lankan government did not prosecute him.

The rights watchdog said Tamil Tiger forces under Karuna's command were directly involved in some of the worst crimes of Sri Lanka's ongoing civil war, including torture, summary execution, and use of children as soldiers.

"The British government had an alleged war criminal in custody for six months and couldn't manage to file charges," said Brad Adams, Asia director at Human Rights Watch.

"This was a rare opportunity to hold a leader of the Tamil Tigers accountable for horrific human rights abuses, and the British government blew it.

Immigration authorities in the UK arrested Karuna on November 2, 2007. After a criminal conviction, he served half of a nine-month term for possessing illegal documents. Despite assistance from NGOs and others, the government on May 9 announced that the Crown Prosecution Service found there was insufficient evidence to convict Karuna for any criminal offences in the UK, HRW said.

The British government, HRW noted, had frequently raised concerns about Sri Lanka's deteriorating human rights situation with Colombo and has long criticised the LTTE for serious human rights abuses.

British law permits the prosecution of individuals for serious violations of international law, including torture and war crimes, committed abroad. In 2005, UK courts convicted a former Afghan warlord, Faryadi Sarwar Zardad, for acts of torture and hostage-taking in Afghanistan.

The HRW said in June 1990, some 400 to 600 police officers who surrendered to the LTTE were bound, gagged, and beaten. The Tamil Tigers, including forces under Karuna's control, then executed the Sinhalese and Muslims among them.

"Karuna's escape from justice in the UK is a failure for international justice," Adams said. "Now that Karuna is back in Colombo, the spotlight is on the Sri Lanka government to do the right thing or be deemed complicit in his crimes."