"They (British officials) have asked us to increase security around the embassy and it was increased from last night," Foreign Ministry spokesman Hamid Reza Asefi told a news conference on Sunday.
British diplomats said the petrol bomb was thrown at the back gate of the embassy compound in central Tehran on Friday, leaving scorch marks on both sides of the gate. No one was hurt in the incident.
Asefi said he was unable to confirm the attack had taken place, but added: "Some suspicious people want to show that the country is not stable. In future, if this kind of incident happens in the country it will be seriously confronted."
Iran is generally considered to be a low security risk in the Middle East and attacks on Western embassies, companies or individuals are rare.
But the British embassy has been the target of a number of attacks this year including four shootings at embassy buildings in August and September.
No one was injured in the shootings and Iran announced in October that it had arrested two men who confessed to carrying them out.