The vessel, originally called Grace I and flying a Panamanian flag, set off from the Gulf in April and sailed around the Cape of Good Hope before Gibraltar security forces, aided by British Royal Marines, intercepted it off the coast of the British overseas territory on 4 July.
USA national security adviser John Bolton on Friday alleged that the tanker, which had set sail after its release flying the Iranian flag and renamed the Adrian Darya, had arrived at the Syrian port of Tartus.
It said Britain would raise the issue at the United Nations, with Foreign Secretary Dominic Raab warning that it was part of a "pattern of behaviour. created to disrupt regional security".
Iran's foreign ministry has said they they have sold oil without identifying to which country.
India has been Iran's key trade partner and the second largest customer of Iran's oil, importing 457,000 barrels of Iranian oil a day before the USA withdrawal from the landmark Iranian nuclear deal in May 2018.
In a statement, the foreign office said Iran's actions represented an "unacceptable violation of worldwide norms" and that the United Kingdom would raise the issue at the UN General Assembly later this month.
"Iran has shown complete disregard for its own assurances over Adrian Darya 1", British Foreign Secretary Dominic Raab said on Tuesday.
"Iran's actions represent an unacceptable violation of global norms", the statement said.
The United States will sanction whoever buys Iranian oil, warned the U.S. Treasury Department's top official for countering terrorist financing.
The Iranian Government in Tehran subsequently committed in a meeting in London with Chief Minister Fabian Picardo and in writing in a formal diplomatic note verbal that the vessel and its cargo, if released by Gibraltar from its detention in British Gibraltar territorial waters, would not be provided to any European Union sanctioned entity.
Two Iranian officials and one diplomat told Reuters on August 25 that Iran wants to export a minimum of 700,000 barrels per day of its oil and ideally up to 1.5 million bpd if the West wants to negotiate with Tehran to save the nuclear deal.
The Trump administration past year unilaterally pulled out of a 2015 nuclear deal between Tehran and Western powers, and is pursuing a campaign to bring Iran's exports to zero.