By Emma Pinch

Fears a vengeful motorist is waging a letter bomb campaign heightened last night after a second attack in two days on companies linked to the administration of motoring fines.

Counter-terrorist officers are investigating whether the latest blast, which injured two men, is connected to a series of letter bombs sent to companies, including one in the West Midlands, last month. The latest explosion happened yesterday morning when a package blew up at the offices of Vantis, an accountancy firm linked to Speed Check Services, the provider of digital speed cameras to the police.

Two men in the Vantis offices, on an industrial estate in Wokingham, Berkshire, suffered minor blast wounds to their hands and upper bodies.

On Monday, a bomb in a Jiffy bag exploded in the hands of a mail room worker at the central London offices of Capita, which runs London’s congestion charge system. She was hospitalised.

Scotland Yard’s counter-terrorism unit and Thames Valley Police are trying to establish whether there are any links.

The Thames Valley force is looking to see if there is any connection to three letter bombs sent to companies in Oxfordshire and the West Midlands. Police initially believed it was a "strong possibility" those were sent by animal rights extremists.

In each case, the companies received an A5 jiffy bag containing a crude firework explosive. 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.

The first device, sent to a company in Abingdon that deals with cell research, ignited as a woman opened mail. The second, sent to a company in Culham, failed to ignite. A third was addressed to a firm, not identified by police, in the Chelmsley Wood area of Solihull. It failed to explode.

Thames Valley Police said it was "too early" to say whether there was a link with the latest explosions. Referring to the Vantis explosion, a police spokesman said: "The investigation will now focus on forensic examination."

A 30-metre cordon was set up around the Vantis building and 14 staff evacuated as forensic teams moved in. Company bosses, with employees, spent the afternoon at a nearby pub, The Two Ploughs.

A Vantis spokesman said the company had no direct involvement in the administration of speeding fines and the package did not appear to have been targeted at Vantis. It is understood to have been addressed to a different company.

"We are not directly involved in speeding fines," the Vantis spokesman said. "We do not do any administration of such things. It is not our business.

"Our business is, we are accountants, tax and business advisers. We give advice to small businesses."

He said the company had no connection with Capita, adding: "I think it needs to be made clear we act for lots of companies and lots of companies have their registered offices at our office."

Speed Check Services describes itself as one of the UK’s "foremost providers of intelligent transport solutions for the road traffic market sector".

It is the sole provider of digital speed enforcement technology to police and highways authorities. A spokeswoman said the company was a Vantis client but said police had not indicated it was the intended target.