The Iranian ambassador has been summoned to the Foreign Office over the "complete disregard" of assurances that the Adrian Darya 1 tanker released last month from Gibraltar would not deliver oil to Syria.
The Foreign Office said it was clear the ship's cargo had been transferred to the Assad regime in an "unacceptable violation" of international norms.
The tanker, previously called Grace 1, was seized in July in an operation by British Royal Marines but was released following written guarantees the oil would not be sold to Syria.
Foreign Secretary Dominic Raab said: "This sale of oil to Assad's brutal regime is part of a pattern of behaviour by the Government of Iran designed to disrupt regional security."
The detention of the Adrian Darya 1 last month saw Iran later seize the British-flagged oil tanker Stena Impero, which remains held by the Islamic Republic.