The blast happened during rush hour while the bus stopped to pick up passengers in the city's southern suburb of Piliyandala.

A government spokesman blamed the Tamil Tiger rebel group for the attack and considered it a “cowardly attack on civilians”.

The attack comes as government forces continue a campaign against the Tigers in territory they control in the north of the island.

Violence in Sri Lanka has intensified since January, when the government pulled out of a ceasefire agreement.

This is the first attack where there has been major loss of life since April 6, when a suicide bomber killed 14 people, including a government minister, in the western district of Grampaha.

In February, two separate attacks on buses killed at least 20 in northern Sri Lanka and injured 18 people in a Colombo suburb.

At least 70,000 people have died since the civil war began in 1983.