Britain accused Iran on Tuesday of breaching its assurances that a tanker held off Gibraltar this summer would not transport oil to Syria, and summoned the Iranian ambassador to protest.

"It is now clear that Iran has breached these assurances and that the oil has been transferred to Syria and (President) Assad's murderous regime," the Foreign Office said, adding that it would raise the issue with the UN.

Britain had seized the tanker Grace 1 off Gibraltar on July 4 on suspicion of shipping oil to Syria in breach of international sanctions.

The tanker was released in mid-August after Gibraltar's government said it had received written assurances from Iran that it would not be headed for countries "subject to European Union sanctions".

"There are no longer reasonable grounds to suspect that the detention of the Vessel is required," chief minister Fabian Picardo said in a statement at the time.

Hours before the announcement, the US launched an unsuccessful last-minute legal move demanding that the British overseas territory detain the ship. 

The Grace 1 carried 2.1 million barrels of Iranian oil.

Tehran had retaliated by seizing a British tanker, the Stena Impero, on July 19 in the strategic Strait of Hormuz. It said on Sunday that the tanker would be released shortly.