Gibraltar's Supreme Court allowed the release of an Iranian supertanker on Thursday that was seized last month on suspicion of shipping oil to Syria in breach of EU sanctions, despite a last-minute U.S. request to detain the vessel.

Chief Justice Anthony Dudley said that since Iran had guaranteed in writing that the Grace 1 "was never destined to an EU sanctioned entity... there are no longer reasonable grounds to suspect that the detention of the Vessel is required." 

Iranian ambassador to Britain Hamid Baeidinejad confirmed that Gibraltar released the Iranian supertanker Grace 1 on Thursday. 

"Following a decision by the Gibraltar's authorities and a ruling of a (Gibraltar's) court, the illegal detention of the Iranian oil tanker was lifted," Baeidinejad tweeted. 

Over the past 40 days, Iranian officials held continuous political, legal and technical negotiations with the British side for the lift of this "illegal detention," said the ambassador. 

Preparations have been made for the departure of the Iranian tanker from the Gibraltar region "soon," he added. 

Hours before the announcement, the U.S. launched a last-minute legal move demanding that the British overseas territory detain the ship. 

The move had delayed the court decision on the vessel's fate but Judge Dudley said during his ruling that he had not received a written request from U.S.

"The U.S. desperate attempts to stop the release of the oil tanker at the last moment was a humiliating failure," the Iranian ambassador noted.

The Grace 1, carrying 2.1 million barrels of Iranian oil, was seized by Gibraltar police and British special forces on July 4, provoking a diplomatic crisis. 

It was suspected of carrying the crude oil to war-torn Syria in violation of separate EU and U.S. sanctions. 

Tehran retaliated by seizing a British tanker, the Stena Impero, on July 19 in the strategic Strait of Hormuz – the conduit for much of the world's crude – for breaking "international maritime rules".