Companies and office workers are being advised to take extra care when handling mail after police said a series of letter bombs had injured six people across Britain.
Detectives said the bombs appeared to be designed to shock rather than kill and said the motive was not yet clear.
"If companies have in place postal screening measures, we ask them to make the best use of them," said Anton Setchell, the national coordinator for domestic extremism.
"If they have any suspicions about any letter or package, they should leave it unopened and call their local police immediately.
The warning came after a blast yesterday at the Driver and Vehicle Licensing Agency (DVLA) in Swansea, south Wales, which was the third in three days aimed at agencies or contractors involved in enforcing traffic rules.
Police also revealed that another bomb had hit the director of an unspecified company at his home on Saturday.
Media reports have speculated that disgruntled motorists could be to blame. However the bombs were similar to three others sent in January, one of which bore the name of a jailed animal rights activist.
All seven bombs were in padded envelopes.
"There has been mention of a connection to animal rights extremists, and to groups that hold grudges around motoring and transport issues," Mr Setchell said.
"We have had no contact from any group or organisation claiming responsibility or setting out their reason why this course of action is being taken."
A man described as the victim of Saturday's attack in southeast England, who gave his name only as Mike, told BBC television he had received the mail bomb at home but now believed it was targeted at his business, which he did not name.
"I just opened the envelope ... I pulled it and - bang," he said. "Fortunately ... I only got a few minor injuries in the stomach, a bit of ash in the face, and burnt finger and thumb."
A letter bomb exploded on Tuesday at a company that handles mail for a firm that makes traffic regulation devices, injuring two men. On Monday, a woman was injured by a letter bomb at the headquarters of a firm managing London's congestion charge, which bills motorists entering the city centre.
On January 18th, two incendiary devices were sent to businesses near Abingdon, central England, one of which exploded, and another was found at a company in Birmingham the following day.