Britain's chief police officers issued a public warning on Wednesday afternoon that parcel bomb explosions "could have been more serious" in the coming days.

In a press briefing in London, Anton Sechell, the national coordinator for domestic extremism, said: "The packages received so far have caused minor injuries, but could have been more serious".

"I am appealing today for companies, organizations and individuals to take extra care when handling mail," he said.

He issued the warning after revealing that seven letter bombs have been received across the country in the last three weeks and nine people have suffered minor injuries in the attacks.

Four people were hurt in a parcel bomb explosion on Wednesday morning at the Driver and Vehicle Licensing Agency center in Swansea, Wales.

Earlier, the police in Kent, southeast England, confirmed that a letter bomb exploded last Saturday. It was sent to the private address of a 53-year-old man in Folkstone, and he was slightly injured by the blast.

Britain's Prime Minister Tony Blair expressed his sympathy on Wednesday to those caught up in a series of parcel bomb attacks during the past weeks.

In the House of Commons debate, Blair said the investigation on the attacks is under way.

Home Secretary John Reid said that a series of parcel bomb attacks are worrying, and he was being kept fully briefed on the investigation.

Police are investigating links to the spate of parcel bombs sent to organizations involved in running Britain's roads, Sky News Television reported.

On Tuesday, two men suffered blast wounds to their hands and upper bodies in a package explosion at the offices of an accountancy firm linked to Speed Check Services in Wokingham, Berkshire, south England.

On Monday, a female worker was hurt by a device at the Offices of Capita in central London, which runs London's congestion charge system.

The attacks appeared to confirm fears that an enraged motorist is mounting a daily parcel bomb campaign.

Editor: Yan