Police chiefs issued a public warning over the letter bomb campaign today as they revealed that seven explosive devices have been posted in the last three weeks.
The Association of Chief Police Officers also revealed that detectives have established a link between three of the devices.
They are thought to be the three letter bomb attacks in consecutive days this week on motoring related companies in London, Berkshire and Swansea.
The latest blast happened at the Driver and Vehicle Licensing Agency's main centre in Swansea this morning and left three women injured.
It raised fears that an enraged motorist is waging a letter bomb campaign against traffic enforcement organisations across the country.
It also emerged today that police are investigating similarities with another letter bombing at a house in Folkestone, Kent, at the weekend which injured a 53-year-old man.
It was disclosed yesterday that three letter bombs were sent to companies in Oxfordshire and the West Midlands area last month.
An investigation into possible links between the seven letter bombs is being co-ordinated by Acpo's National Co-ordinator for Domestic Extremism (NCDE) - Assistant Chief Constable Anton Setchell.
Mr Setchell said today: "The packages received so far have caused minor injuries, but could have been more serious.
"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."
Acpo issued its warning to companies, organisations and members of the public after what it described as the recent posting of "viable explosive devices".
"Seven devices have been received in the last three weeks and six people have received minor injuries," a spokeswoman said.
The latest blast prompted Prime Minister Tony Blair and Home Secretary John Reid to publicly express their concern over the bombings.
Today's explosion followed yesterday's blast at the offices of 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.
On Saturday, a letter bomb exploded when it was opened by a man at a house in Folkestone.
"Detectives are currently making inquiries to determine who sent the letter and why," Kent Police said.
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 in 2001 while serving an 18-year jail sentence for a firebombing campaign, was named on the back of one envelope.
At Prime Minister's Questions today, Mr Blair expressed sympathy to all those caught up in the "traumatic" attacks and said they were being investigated "very closely".
Speaking outside the Home Office this morning, Mr Reid said the letter bombings were clearly "a cause for concern".
"Naturally, these incidents are worrying," he said. "The police are on top of this. They are keeping me informed here."
Fears that a disgruntled motorist is behind the most recent attacks were apparently confirmed by this morning's explosion at the DVLA.
A South Wales Police spokeswoman said: "Three people are being treated for injuries sustained as a result of the incident.
"One woman has sustained minor burns and two other women have sustained hearing-related injuries.
"There is no intelligence to suggest that anyone else is currently at risk and we would appeal for calm. However, workers in relevant posts are urged to be vigilant and carry out work with increased caution."
All incoming mail to the DVLA has been stopped in the wake of the blast.
The DVLA is an executive agency of the Department for Transport.
The Agency's primary aim is general motoring law enforcement - maintaining registers of drivers and vehicles, and collecting car tax.
An army bomb disposal squad escorted by two police cars arrived at the DVLA at 1pm today.
The Royal Logistic Corps bomb disposal team was waved through the police cordon and entered the building's compound.
A South Wales Police spokesman said there had been no new developments.