British Foreign Secretary Jeremy Hunt said Saturday that Tehran's seizure of a UK-flagged oil tanker showed "worrying signs Iran may be choosing a dangerous path of illegal and destabilising behaviour".
Iran's Islamic Revolutionary Guard Corps said Friday it had seized the Swedish-owned Stena Impero in the Gulf's Strait of Hormuz for breaking "international maritime rules".
The incident came hours after a court in the British overseas territory of Gibraltar said it would extend by 30 days the detention of Grace 1, an Iranian tanker seized two weeks ago in an operation aided by British Royal Marines on allegations of breaching EU sanctions against Syria.
Hunt said on Twitter that seizing the Stena Impero showed "worrying signs Iran may be choosing a dangerous path of illegal and destabilising behaviour after Gibraltar's LEGAL detention of oil bound for Syria.
"Our reaction will be considered but robust. We have been trying to find a way to resolve Grace 1 issue but WILL ensure the safety of our shipping."
British cabinet minister James Brokenshire said the seizure of the tanker was "completely intolerable" and said London was still seeking to establish diplomatic connections with Tehran over the incident.
"The actions of the Iranians are completely unacceptable. It is so important that we maintain this free navigation through the Gulf," the housing secretary told BBC radio.
"We want to see this matter resolved in a diplomatic way. The Iranians need to release this vessel as quickly as possible."
David Richards, the professional head of Britain's armed forces from 2010 to 2013, said Britain's options were "pretty limited" in what military action it could take without the support of allies such as the United States, should economic sanctions fail to resolve the situation.
"The Royal Navy, if you're looking at that in the first instance, is just too small to have a significant effect without being with allies," he told BBC radio.
The Stena Impero had been heading for Saudi Arabia on Friday when it collided with a fishing vessel, according to authorities at the southern Iranian port of Bandar Abbas, off which the tanker is now anchored.
Tom Tugendhat, who chairs parliament's cross-party foreign affairs scrutiny panel, said military options would now be "extremely unwise".
"If it has been taken to Bandar Abbas then that's an important Iranian military port and I think any military options will therefore be extremely unwise," the MP told BBC radio.