Britain sent a letter to the Security Council saying that "Iranian forces approached the tanker in the territorial waters of the Sultanate of Oman, the Iranian action against the tanker constitutes an illegal interference."
Britain added in its letter that the threat of navigation in international corridors is unacceptable and represents a major escalation, and that current developments are very worrying and give priority to calm.
The Daily Telegraph reported earlier that British ministers were putting plans to impose sanctions on Iran after Iran seized a British-flagged oil tanker in the Gulf.
British Foreign Secretary Jeremy Hunt is expected to announce diplomatic and economic measures on Sunday, including the possible freezing of assets in response to the incident, the newspaper said.
The British Foreign Office said on Saturday that London summoned the charge d'affaires in London, after the detention of a British flag carrier in the Strait of Hormuz.
Iran seized the British-flagged Steina Ampero with 23 men on Friday night and the maritime tracking equipment found it was heading from the port of Fujairah to a port in Saudi Arabia.