The blast came hours after the rebels' top leader blamed the government for a recent escalation of fighting in the more than two-decade-old civil war that has killed an estimated 70,000 in the Indian Ocean island nation.
The suicide bombing targeted the offices of Douglas Devananda, the minister of social services and the leader of the Eelam People's Democratic Party, an ethnic Tamil party considered a rival to the separatist Tamil Tigers, the military said.
Devananda, the target of repeated assassination attempts, was not injured in the attack, the military said. The blast killed one of his staff members and injured two others, one critically, said Dr. Hector Weerasinghe, the medical director of Colombo National Hospital . The bomber was also killed.
"This was an attempt to kill the minister," said military spokesman Brig. Udaya Nanayakkara, blaming the separatist Tamil Tigers.
Rebel spokesman Rasiah Ilanthirayan said he was unaware of the blast. The Tamil Tigers, listed as a terror organization by the United States and the European Union, have carried out more than 240 suicide bombings.
The rebel group has been fighting since 1983 to create a separate homeland for Sri Lanka's minority Tamils following decades of discrimination by governments controlled by the Sinhalese majority.
In addition to attacking government forces and Sinhalese officials, the Tamil rebels routinely target Tamil leaders they perceive as rivals.
The Eelam People's Democratic Party, once part of the rebel cause, renounced violence and joined the political mainstream in 1987.
Devananda has accused the rebels of killing dozens of his party members and supporters even after the guerrillas signed a 2002 cease-fire agreement. In 2004, another suicide bomber targeting Devananda blew himself up while being frisked, killing four police officers.
The woman who attacked the ministry Wednesday, a polio victim, was allowed past the gate and into the ministry complex with other members of the public who had gathered for the minister's weekly open meeting to discuss their problems, Nanayakkara said.
When Devananda's security personnel searched her at the door about 8:05 a.m. , she detonated her bomb vest, he said.
After the blast, dozens of soldiers armed with rifles sealed off the compound and set up roadblocks on the streets outside. The military barred journalists from the scene, but released pictures showing a damaged entryway filled with shattered glass and blood.
The suicide attack came a day after 11 schoolchildren and two adults were killed when Sri Lanka 's military detonated a roadside bomb near their vehicle as it traveled inside the Tamil Tiger's stronghold in northern Sri Lanka , the rebels said.
Nanayakkara denied the military was behind the blast.
Air force jets also dropped 12 bombs on the Tamil Tigers' radio station Tuesday, just an hour before it was to broadcast an annual address by rebel leader Velupillai Prabhakaran. The airstrike killed five station employees and four local residents, a rebel statement said. The military confirmed the attack.
In his speech, made from an undisclosed location, the reclusive Prabhakaran blamed the international community for the worsening violence, saying military, economic and diplomatic assistance to the government was feeding the bloodshed.
"This partisan and unjust conduct of the international community has severely undermined the confidence our people had in them. And it has paved the way for the breakdown of the cease-fire and the peace efforts," the rebel leader said, according to a copy of the address e-mailed to journalists.
He also accused the government of continuing "on the path of violence. It only desires to find a solution to the Tamil question through military might and oppression," he said.
Also Wednesday, the military said one rebel was killed in fighting in northern Sri Lanka on the previous day.