British police have issued a public warning over a letter bomb campaign to workers across the UK after it emerged that seven explosive devices have been posted in the last three weeks.

The latest blast at the Driver and Vehicle Licensing Agency in Swansea in Wales prompted British Prime Minister Tony Blair to publicly express concern at the bombings.

A total of eight people have been injured in the attacks and it is feareda that someone could be seriously injured or killed if the bomber is not caught soon.

The first three letter bombs were sent to companies in Oxfordshire and the West Midlands area last month.

In each case, the companies received an A5 padded bag containing a crude firework-type explosive device.

Each had a similar return address, while Barry Horne, an animal rights extremist who died while serving an 18-year jail sentence for a firebombing campaign, was named on the back of one envelope.

On Saturday, a letter bomb exploded at a house in Folkestone in Kent, injuring the 53-year-old man who opened it.

This week has seen three letter bomb attacks in consecutive days on motoring-related companies in London, Berkshire and south Wales.

The blast in Swansea this morning left three people injured.