Two letter bomb attacks in Oxfordshire are part of a major police probe after a similar device was sent to the DVLA headquarters in Swansea yesterday.

Seven mail bombs have been sent out in the past three weeks - including two to Abingdon and Culham - and Oxfordshire firms and organisations have been told to be extra vigilant.

The police's National Co-ordinator for Domestic Extremism indicated that both animal rights extremism and the possibility of a motorist with a grudge were being examined.

Assistant Chief Constable Anton Setchell confirmed that all seven devices were of a "small, pyrotechnic nature" designed to shock rather than kill.

Mr Setchell said: "We are at an early stage of our investigation and I want to keep a completely open mind about the motivation.

"However, there are clearly some priority lines of inquiry."

He added the injuries suffered so far could have been more serious and he appealed for people to take extra care when opening mail - and to call the police if they were worried.

All seven have targeted businesses and Government agencies.

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

Staff at the Banbury-based Thames Valley Safer Roads Partnership, which runs Oxfordshire's speed cameras, were told to be on alert. Ian Wilson, a data manager, said: "In the light of recent incidents, an email has been sent to staff to urge them to take care with any suspicious packages or parcels.

"Staff are being told that if they have any concern over a suspicious package, they should not open it, and should call police."

Staff at Capita Education Services in Cumnor Hill, Oxford, were also on alert following a letter bomb attack on Monday at the offices of Capita in Victoria, central London, which runs London's congestion charge system.

A woman post room worker needed treatment for injuries to her hands and stomach.

Spokesman Caroline Mooney said the Cumnor office was one of 76 different businesses attached to the group and added: "We are fully supporting police inquiries and are working with the Royal Mail to review and improve post-handling procedures group-wide."

Yesterday, one worker was injured when a letter bomb exploded in the post room at the Driver and Vehicle Licensing Agency centre in Swansea.

On Tuesday, a package exploded at the offices of an accountancy firm Vantis, on an industrial estate in Wokingham, Berkshire. Vantis is linked to Speed Check Services, the provider of digital speed cameras to the police.

Two staff, both in their 30s, suffered minor blast wounds.

Thames Valley Police is liaising with the Metropolitan Police on possible links between the Wokingham and Capita incidents.

A police spokesman said it was too early to say whether the two Oxfordshire incidents and another in the West Midlands were linked to this week's letter bombs.

Two of the letters bore the name of Barry Horne, the animal rights activist who died in 2001, and police said at the time that they believed the bombs had been sent by animal rights protesters.

Prime Minister Tony Blair told the Commons yesterday that attacks were being investigated "very closely".