The device, which was rigged to a timer, was attached to a steel pylon located between a key thermal power generating facility and the main control centre in Colombo.

The high-tension line supported by the pylon forms the backbone of the country's power transmitting system.

The blast was heard five kilometres (three miles) away but caused only minor damage to the pylon, and no disruption to power supplies, officials said. No one was hurt in the explosion.

"The line is energised and there is no immediate threat of the cables collapsing," a spokesman for the Ceylon Electricity Board said. "We have not switched off the supply, but we will start repair work soon."

The defence ministry blamed the Liberation Tigers of Tamil Eelam (LTTE), who have been fighting for an independent Tamil state in the north and east of the island since 1972.

Engineers said the bomb could have destabilised Sri Lanka's entire electricity network by disconnecting the thermal generators from the national grid.

Sri Lanka is heavily dependent on the generators due to a drought that is affecting hydroelectric power stations.

"We don't want to openly discuss the transmission and distribution networks, but obviously the attackers knew what kind of damage they could have caused," one engineer said.

Authorities have begun a major search in the area to track down any more explosives and patrols were also stepped up along the cable route, police said. Military bomb disposal units have also been dispatched.

The blast came as the capital was on high alert for Tamil Tiger bomb attacks.

On Sunday, Prime Minister Ratnasiri Wickremanayake warned that Tamil Tiger rebels had rigged up a truck with over a tonne of explosives that could blow up half of Colombo, a city of over 600,000 people.

Meanwhile, Britain Monday asked its former colony Sri Lanka to end its military campaign and resume peace talks with separatist Tamil Tiger guerrillas.

Britain's junior foreign minister, Kim Howells, said here that London was worried about growing rights abuses and an escalation in the conflict, which has left scores of combatants dead on both sides.

"The situation in Sri Lanka looks darker now than my last visit in February, where there were many hopes of an all-party political solution that would create a basis for future talks. That looks further away now," Howells told reporters.