There was no immediate claim of responsibility for the attack, which appeared similar to a bombing outside the same offices in August. The perpetrators of the August blast have not been identified.
"It may be that today's bombing ... was a response to the anti-Iranian stance taken by some countries," Deputy Interior Ministry Ali Ahmadi told the ILNA labour news agency.
In Tehran, police spokesman Mehdi Ahmadi said a hand-made bomb had exploded, breaking windows but not hurting anyone.
An employee at the building in a busy Tehran street said the device had gone off in a rubbish bin near a 10th-floor lift.
A British Airways spokesman in London attributed the blast to a "percussion device ... designed to create more noise than damage".