The BBC's Susannah Price: "Tamil Tigers still see violence as the way forward"

The MP, Neelan Thiruchelvam, of the Tamil United Liberation Front (TULF), was in his car near his office when a man threw himself at the vehicle, detonating an explosion which killed them both.

The Tamil Tigers are suspected of carrying out the attack.

An assistant to Neelan Tiruchelvam in shock after the blast

Five others were injured in the blast, including two of the MP's bodyguards.

The attacker arrived on a motorbike in heavy traffic before dismounting and approaching the politician's car, police said.

However, the police say that contrary to earlier reports, the motorcyclist originally implicated was not involved and that the bomber was acting alone.

Attack condemned

The Sri Lankan cabinet has issued a statement expressing its grave sorrow and sadness over the killing of Mr Thiruchelvam.

The attack has also condemned by Sri Lanka's National Peace Council.

In one of the first foreign reactions, the US embassy in Colombo said: "We express our profound regret and strongly condemn the violent attack that claimed his life."

Mr Thiruchelvam's TULF party said his killing highlighted the need for a settlement of the Sri Lankan conflict.

"The tragic death...once again emphasises the urgent need to evolve a peaceful political resolution to the country's national conflict," a TULF statement said.

Hallmarks of Tamil Tigers

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Police at the scene refused to speculate who was responsible, but the killing bore the the hallmarks of the rebel Liberation Tigers for Tamil Eelam, or Tamil Tigers.

The group has been fighting a long-running war for a separate homeland for the Tamil minority in the north and east of the island.

Around 58,000 people have been killed in more than 16 years of conflict.

Mr Thiruchelvam's party supports the government, making it a target for the Tigers.

Mr Thiruchelvam also headed the International Centre for Ethnic Studies, a think-tank focussing on the ethnic strife between the Sinhalese majority and Tamil minority.

Government autonomy package

As a lawyer, he had helped the government draw up plans for devolving power to Tamil areas in the north and east, which President Chandrika Kumaratunga hopes will erode support for the Tamil Tigers.

However, BBC Colombo correspondent Susannah Price says that following Mr Thiruchelvam's death and reports of warnings sent to other Tamil MPs, it is unclear how many MPs will openly support the government package.