Iranian authorities impounded the Stena Impero with 23 crew members aboard after patrol boats of the Islamic Revolutionary Guard Corps seized it Friday in the highly sensitive Strait of Hormuz.
Iranian officials said the tanker had turned off its transponder after hitting an Iranian fishing boat and was detained after failing to respond to distress calls.
But in a letter dated Saturday, British charge d'affaires Jonathan Allen rejects each of the Iranian claims.
In the letter to Security Council president Gustavo Meza-Cuadra, Allen says the Stena Impero's transponder was switched on, that the ship was approached in Omani waters, and that it was "exercising the lawful right of transit passage in an international strait."
As to the claim that the tanker had collided with an Iranian fishing boat, Allen says: "There is no evidence of this. Even if it had occurred, the ship's location within Omani territorial waters means that Iran would not have been permitted to intercept the Stena Impero."
Following that incident and another involving the Liberian-flagged but British-managed tanker Mesdar -- boarded by Iranian forces but then released -- London on Friday asked all British-flagged ships to avoid the Strait of Hormuz "until further notice."
Allen's letter says British officials "do not seek confrontation with Iran. But it is unacceptable and highly escalatory to threaten shipping going about its legitimate business through internationally recognized transit corridors."
He called on Iran to release the tanker while diplomatic efforts to resolve the situation continue.
"Our priority," he said, "is to de-escalate."