A letter bomb exploded Monday in the mailroom of a London company that controls the capital's traffic congestion charge, slightly injuring a female worker, police said.

The padded envelope exploded at an office belonging to Capita PLC, which administers the $16 daily fee meant to cut down on traffic in central London and collects television licensing fees. It also developed the database for the Criminal Records Bureau, combining data from 43 British police departments, and has significant contracts with government departments.

"We can confirm that there has been a small explosion at our Victoria Street office this morning," said a Capita spokeswoman, speaking on condition of anonymity in line with department policy.

Scotland Yard said the injured woman was a Capita employee.

Capita was formed in 1984 and has more than 26,000 staff in the U.K., the Channel Islands, Ireland and India.

"We didn't hear a bomb, but there were a lot of ambulances — maybe three or four — and the fire brigade," said a woman who works at a nearby hair salon who asked not to be named. She said the injured woman was crying, had a bandage on her arm and another on her midriff.

The office is near Scotland Yard police headquarters and several other government offices.

The Irish Republican Army used letter bombs in the early 1970s as part of its campaign to end British rule in Northern Ireland, targeting numerous British government offices. The tactic maimed dozens of people but killed nobody.

The Royal Mail sought to prevent larger explosive devices from entering the post by erecting barriers on letter boxes that reduced the size of the slits, so that only thin envelopes could be inserted.

In larger packages, the major Royal Mail sorting offices since the 1980s have deployed X-ray scanners to detect suspicious battery-powered objects before they could reach their intended target. The major sorting center in Belfast, for example, has intercepted scores of such homemade devices using the technology.