JAKARTA - THREE militants awaiting execution for the 2002 Bali bombings plan to challenge the legality of death by firing squad, but it is unclear if the move will lead to a last-minute stay of execution, their lawyer has said.

Mr Mahendradatta, who goes by a single name, said he would file a petition next week at the Constitutional Court to argue that convicts who went before a firing squad sometimes did not die immediately, causing unnecessary suffering.

'We're seeking an execution without pain,' said the lawyer, who in the past had argued that his clients would prefer to be beheaded.

Ali Ghufron, Imam Samudra and Amrozi Nurhasyim were convicted in 2003 of planning and helping to carry out the Oct 12 suicide bombings targeting two packed clubs on Bali island, killing 202 people, mostly tourists.

The three, who have admitted taking part in the attacks, already have exhausted the appeals process.

A spokesman for the Attorney-General's office, Mr Bonaventura Daulat Nainggolan, declined to say whether the petition would result in an automatic last-minute stay in their executions, which the authorities said would likely be carried out before September.

Mr Mahendradatta acknowledged the executions could well go ahead even if the case was lodged at the Constitutional Court, but he said it would be 'murder' if they did.

Meanwhile, police have launched an investigation after a cache of up to 10 petrol bombs was found stashed under a train heading to Jakarta.

ASSOCIATED PRESS, REUTERS