The oil tanker, which has been the center of a diplomatic row between the UK and Iran since it was seized by Royal Marines off the coast of the British Overseas Territory of Gibraltar, is set to sail on Sunday night.

The Iranian Ambassador to the UK confirmed with a tweet, saying that "with the arrival of two specialized engineering teams to Gibraltar... the vessel is expected to leave tonight (Sunday night)."

The news came after the Gibraltar authorities turned down a final request by the United States to retain the Adrian Darya 1, which is the new name of the tanker that was originally named Grace 1.

The ship was boarded and taken to Gibraltar last month on suspicion of attempting to carry oil to Syria in "breach of EU sanctions" -- charges denied by the Iranian government.

Gibraltar's Chief Minister Fabian Picardo said on Thursday the vessel would be released after he had received assurances that the tanker's cargo would not be taken to Syria.

He added that if the US decided to present a late legal challenge to the decision, authorities in Gibraltar would make "an objective, legal determination of that request."

Gibraltar on Sunday rejected the US request on the grounds that "the EU sanctions regime on Iran is fundamentally different to that of the United States".