The British foreign ministry says Iran "repeatedly gave assurances to the government of Gibraltar" that the Adrian Darya I, formerly known as Grace I, would not deliver oil to any EU-sanctioned entity in Syria or elsewhere.
Tehran's assurances persuaded Gibraltar to release the tanker about five weeks after it was seized on July 4 by local police aided by British forces.
"It is now clear that Iran has breached these assurances and that the oil has been transferred to Syria and (President Bashar al-Assad's) murderous regime," the ministry said.
It said Britain would raise the "unacceptable violation of international norms" at the United Nations.
After summoning the Iranian ambassador, Foreign Secretary Dominic Raab said Tehran had "shown complete disregard for its own assurances over Adrian Darya 1".
"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," Mr Raab said.
The seizure of the tanker escalated tensions as Iran later seized the British-flagged Stena Impero in the Strait of Hormuz, a strategically important shipping route in the Gulf region.
Washington had warned Mediterranean countries that any co-operation with the tanker would be treated as support for terrorism.
