Emergency services were dealing with a small explosion at 9 a.m. (4 a.m. EST) at a company in Oaklands Business Centre, Wokingham, Thames Valley police said in a statement.
Police said it was "likely" that an item of mail had ignited and caused the "small explosion."
It was too soon to say whether the explosion was linked to a letter bomb that detonated in central London Monday, injuring one person, police said.
On Monday, a padded envelope exploded at an office belonging to Capita Group PLC, which administers the $16 daily fee for vehicles meant to cut down on traffic in central London and collects television licensing fees.
Vantis PLC, a tax and accounting company, confirmed it was targeted in Tuesday's attack. Vantis also handles government contracts.
Army bomb-disposal units surrounded the company's office, which is part of business park comprised of red-brick office buildings.
Two ambulances and a crew from Royal Berkshire Fire and Rescue Service were also at the scene. Police taped off the road directly outside the building and evacuated staff, but officers and other emergency services workers could be seen walking near the offices.
Workers from neighboring offices said they were still at their desks when police in white suits prepared to enter the building to gather evidence.
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.
In recent years, animal rights activists have also sent letter bombs to people involved in animal testing.