A "percussion device" has exploded outside the offices of BP and British Airways (BA) in Iran's capital, Tehran.
The Sayeh Building houses a number of foreign firms
No-one was hurt by the blast, on the 10th floor of the central city office block, police said. The Foreign Office said it was unclear who was to blame.
BA said: "Our security staff said it was a 'percussion device' - quite similar to a firework - designed to make a lot of noise but no damage."
In August, a bomb exploded in a dustbin at the building, causing minor damage.
A British embassy spokesman in Tehran said Wednesday's device had left "minor blast marks" near the fire escape.
The Foreign Office said embassy staff were in touch with the companies involved in Tuesday's bomb.
A spokesman for BA said: "There was just a big bang and there's some soot on the ground by the fire escape.
"Our staff at the time didn't get evacuated. They stayed at their desks."
The British embassy in Iran said the building also houses a number of other international companies.
The bomb on 2 August exploded in a dustbin outside the BA offices, causing slight damage but no casualties.
The BBC's Frances Harrison said there was an assumption that the attacks were linked to rising tension between Iran and Britain.
Britain led the way at the United Nations Security Council to condemn the Iranian President Mahmoud Ahmadinejad for saying that Israel should be "wiped off the map".
It is also applying pressure on Iran to reveal its nuclear ambitions.