A third letter bomb in three days exploded in Britain on Wednesday in a motoring-related government agency in south Wales, injuring a woman and fuelling theories about a possible link between the blasts.
And as fears of a possible package bomb campaign grew, it also emerged that other, previously unreported explosions had occurred, including one in Kent at the weekend which caused minor injuries.
Home Secretary John Reid voiced concern about the low-level explosions, but warned against unwarranted speculation.
"Naturally, these incidents are worrying...It is important that we allow police to get on with their investigation without undue speculation," he told reporters, in his first comments on the blasts this week.
The new explosion occurred at the Driver and Vehicle Licensing Agency (DVLA) in Swansea, said South Wales police.
A spokesman said the woman was injured by "a small letter-type explosion" shortly after 9 a.m. and was taken to hospital.
"Police are in attendance following an incident at the DVLA offices in Swansea, where a small explosion has been reported," said a police statement.
"One female has been taken to hospital with injuries, which are not believed to be life-threatening. A cordon has been put in place and nearby residents are being evacuated as a precautionary measure."
The incident came after a suspect package exploded on Tuesday in Wokingham, west of London, at the offices of a company dealing with speeding fines, and a blast on Monday in London, at a firm dealing with the London congestion charge.
On Wednesday, it emerged that a letter bomb exploded at a private address in the southeastern coastal town of Folkestone, Kent, on Saturday, slightly injuring a householder.
Police have made no formal link between the blasts, but the British press has speculated about the possibility of a disgruntled motorist targeting authorities who control the roads in Britain.
"Detectives will examine any similarities between the incidents. At this stage we can't say whether any of them are linked," said a Kent police spokesman.
Monday's blast occurred at the offices of outsourcing firm Capita Group Plc in London, near the Metropolitan Police's New Scotland Yard headquarters.
Among other projects, the firm manages the British capital's controversial congestion charge, under which motorists pay a daily fee for using roads in the city centre or face fines.
On Tuesday, two men were injured in an explosion at the offices of Vantis Plc in Wokingham, 60 kilometres west of central London.
The firm deals in business services, but press reports indicate that the device was addressed to a specific contractor who deals with managing speeding fines.