A woman employee at the company which runs London’s congestion charge was being treated in hospital last night after a letter bomb exploded in her hands.

The bomb, which was stuffed in a jiffy bag, detonated as she opened it in the postroom of Capita’s main offices in central London. The force of the blast left her with injuries to her hands and stomach.

Witnesses described how she was led away from the building in tears, with one of her arms bandaged and bleeding from her wounds. One said it looked initially as though she had been stabbed. It is understood that the bomb was not intended for her. It was apparently addressed to someone specific in the company, but police were last night refusing to disclose the identity of its intended target.

Scotland Yard’s counter terrorism unit is investigating the explosion, which happened at around 9.40am at Capita’s offices in Victoria Street, less than a mile from the Houses of Parliament.

With London still on a high state of terror alert, all six floors of the offices were evacuated and a cordon put in place around the building while bomb disposal experts worked at the scene.

Some witnesses described it as being akin to a "war zone", with sniffer dogs deployed to check for further devices.

Sources said the motive for the attack was not immediately clear. It is not thought that the company had been receiving any threats and there was no threatening message accompanying the bomb.

Capita, a leading business out-sourcing company, provides HR and payroll services to central and local Government departments, the police, and IT services to schools, among a raft of other services.

Scotland Yard responded to suggestions that the sender of the letter bomb could have been a disgruntled employee, a congestion charge protester or even an animal rights activist by saying that nothing was being ruled out. The bomb itself was being examined by explosives experts last night.

Eyewitness Ben Hetchin described how the injured woman was led out of the building with her hands covered in bandages.

"At first we thought, presumed, it was a fire alert as we get them often, but then a woman was led out with her hands bandaged like a boxing glove, totally covered," he said.

Elaine Rogers, who works at a shoe repair shop next door, said the victim looked to be suffering from "very bad shock" as she was put into an ambulance.

"She was still walking but she was in a lot of shock and she was crying," she said. "She had a wound to the stomach which you could see was still bleeding and she had a wound to the hand which she had up in the air and that was still bleeding, but they’d wrapped it up.

"She was sitting in the ambulance for about 15 or 20 minutes outside the shop, which I was quite shocked about, but she was being interviewed by police so I thought maybe it wasn’t a serious injury."

"The police shut the whole street off. It’s like a war zone where they’ve brought everybody in and they’ve got the sniffer dogs out."