Pado Mahn Shar was a veteran of the Karen People's struggle
The secretary general of Burma's largest rebel group, the Karen National Union, has been killed.
Pado Mahn Shar, who was in his sixties, was shot at his home in the Thai border town of Mae Sot, his family said.
He was targetted by two men in a pick-up truck, while sitting on the veranda of his home. He died instantly.
The KNU and its military wing, the Karen National Liberation Army, have spent nearly 60 years fighting the Burmese government.
They want to establish an autonomous Karen state in the east of Burma, and to protect the Karen people from what they say are abuses by the government.
Despite their fighting spirit, the Karen rebels have suffered a number of recent setbacks, including political splits and defections to the government.
The KNU, through its armed wing the KNLA, has been fighting for greater autonomy since 1949
The KNLA split in half in 1994, with the new group, the DKBA, making a pact with the government
The KNU and the junta reached a 'gentleman's agreement' in 2003, but it quickly broke down
The Karen are just one of many ethnic minorities in Burma. Much smaller rebel groups still exist in the Shan, Karenni and Mon states
Militarily, they have been driven further and further back towards the Thai border.
However, Pado Mahn Shar told the BBC last year: "Whatever happens, we cannot surrender.
"If the government won't offer us a proper peace settlement, we have to carry on fighting."
The BBC's Kate McGeown, who met Pado Mahn Shar on the Thai-Burmese border, says his death will be a serious blow to the Karen rebels.
As it is, they face a huge challenge against the Burmese army, which is vastly superior both in terms of numbers and equipment.
Analysts said it was not clear who was behind the shooting.
It might be the result of rivalry within the Karen rebel movement, Burmese expert Aung Naing Oo told the AFP news agency.
Pado Mahn Shar (l) said the rebels would never give up
Thai police said the gunmen approached Pado Mahn Shar and addressed him in the Karen language before opening fire.
But what the KNU really feared, Aung Naing Oo said, was that the Burmese government might have ordered it.
"Everybody is terrified that there could be a hit list from the Burmese military circulating around Thailand of people to be killed," he said.
Pado Mahn Shar had himself predicted an increase in bloodshed, telling Reuters this week that violence might rise ahead of a constitutional referendum in Burma in May.