Police chiefs issued a public warning about a letter bomb campaign to workers across the country today after it emerged that seven explosive devices have been posted in the last three weeks.

The latest blast was at the Driver and Vehicle Licensing Agency (DVLA) in Swansea this morning.

A total of eight people have been injured in the attacks.

An investigation into possible links between the seven letter bombings is now being led by the National Co-ordinator for Domestic Extremism (NCDE) Assistant Chief Constable Anton Setchell.

Each of the packages contained "viable explosive devices", according to police.

"The packages received so far have caused minor injuries, but could have been more serious," Mr Setchell said. "I am appealing today for companies, organisations and individuals to take extra care when handling mail.

"If they have any suspicions about any letter or package, they should leave it unopened and call the police immediately."

The first three of the seven letter bombs were sent to companies in Oxfordshire and the West Midlands area last month. They are being linked by Thames Valley Police.

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 in 2001 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, 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.

Yesterday, a letter bomb exploded at an accountancy firm linked to Speed Check Services, a provider of digital speed cameras to the police.

That came after a letter bomb attack the day before at the central London offices of Capita, which runs London’s congestion charge system.

Fears that a disgruntled motorist is behind the most recent attacks were apparently confirmed by this morning’s explosion at the DVLA - the agency which maintains registers of drivers and vehicles, and collects car tax.

All incoming mail to the DVLA has been stopped in the wake of the blast. Employees said they had heard it was a small firework device in a Jiffy bag.

"The woman who was injured had bandages around her eyes and lacerations to her chest," one said.

"We are absolutely petrified here. Everyone is really frightened."

A spokeswoman for Morriston Hospital, in Swansea, said four workers from the DVLA had been treated there today.

She said: "A woman is continuing to receive treatment for injuries, which include cuts to her hands and body.

"Her condition continues to be stable, and her injuries are not life-threatening."

The woman, who has asked for her personal details not to be disclosed, gave this statement: "I was shaken, shocked and frightened.

"It’s hard to describe how I felt. Everybody started running around me and I felt quite calm, but I didn’t really know what had happened."

The first of this week’s letter bombs exploded at the offices of business outsourcing company Capita in Victoria Street, central London.

The bomb, stuffed into a padded bag, exploded in the hands of a worker in the postroom, leaving her with blast wounds to her hands and stomach.

She was taken to hospital and is said to be making a good recovery.

Then yesterday, a letter bomb exploded at the offices of accountancy firm Vantis on an industrial estate in Wokingham, Berkshire. The package was not addressed to the company.

It later emerged that one of its clients is Speed Check Services, based in Camberley, Surrey, which is the sole provider of digital speed enforcement technology to police and highways authorities.

That explosion left two men with blast injuries to their hands and upper bodies, but they did not need hospital treatment.

Motoring groups moved swiftly to distance themselves from the bombing campaign.

Nigel Humphries, from the Association of British Drivers, said he was appalled by the attacks.

"I can’t believe anyone would be so stupid. We’re law-abiding people, we do not condone this at all," he said.