Staff at the Banbury firm which runs Oxfordshire's speed cameras have been warned to be vigilant after letter bombs were sent to other organisations linked to motoring fines.

The Thames Valley Safer Roads Partnership is responsible for speed cameras across the region.

Ian Wilson, a data manager for the partnership, said: "We have ensured that all necessary information has been passed on to our staff, and to other staff working with us in this field.

"In the light of recent incidents, an email has been sent to staff to alert them to what has happened and 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."

Today, a woman was injured when a letter bomb exploded at the Driver and Vehicle Licensing Agency centre, in Swansea, South Wales - the third attack on motoring-related sites in consecutive days.

The latest blast happened in the DVLA's post room.

Yesterday, a package exploded at the office of an accountancy firm Vantis, on an industrial estate in Wokingham, Berkshire. Vantis is linked to Speed Check Services, which supplies digital speed cameras to the police.

Two staff, both in their 30s, suffered minor blast wounds to their hands and upper bodies.

That followed a letter bomb attack the day before at the offices of Capita, in London, which runs the capital's congestion charge system.

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

Detectives from Thames Valley Police are liaising with the Metropolitan Police on possible links between the Wokingham incident and the one at Capita.

A police spokesman said it was too early to say whether three incidents last month were linked to the Wokingham and London investigations.

A woman was hurt when a package exploded on January 18, as she opened the morning mail at Orchid Cellmark, a firm specialising in DNA work, in Blacklands Way, at Abingdon Business Park. She was not seriously injured.

Four-and-a-half hours after the Abingdon incident, police were called to LGC Forensics, in Culham, where a second device was found. A third was found in the West Midlands.

Two of the letters bore the name of Barry Horne, an animal rights activist who died in 2001.

Thames Valley Police said at the time that they believed the bombs had been sent by animal rights activists.