Britain has joined the United States and France in sending a flotilla of warships through the sensitive Strait of Hormuz in a pointed message to the Iranian regime.
The Ministry of Defence confirmed that a Royal Navy Type 23 frigate, HMS Argyll, was part of the US-led carrier group to pass through the waterway, as tensions continued to escalate over Tehran's nuclear programme.
The strait, a 34-mile-wide sea passage, connects the petroleum-producing Persian Gulf states to the ocean, making it a strategic choke point on the world's economy.
The EU gave preliminary approval to new sanctions against Iranian oil on Monday. On the table is a total ban on European purchases of Iranian oil - a sanction that would not just hit Iran but key EU buyers including Greece Italy and Spain.
Sanctions are already expected against Iran's central bank.
The full implementation of the sanctions would be delayed until 1 July, due to concerns about their impact on the European economy.
The UK, Germany, France and the Netherlands had been leading calls for nothing more than a three-month delay before the sanctions bite, but Greece, which fears its economic woes will worsen if it cannot find alternative suppliers at Iran-style preferential rates - has urged a much longer phase-in to ease the pain.
The embargo is yet to be formally approved by the EU nations' foreign ministers, who are meeting in Brussels.
In response to the expected sanctions the Iranians have threatened to close the strait - through which 35% of the world's tanker-borne oil exports pass - in retaliation.
It came as Foreign Secretary William Hague urged Iran to "come to its senses" and resume negotiations on its nuclear programme.
Any bid by Iran to block the Strait of Hormuz at the entrance to the Gulf, through which 20% of the world's oil exports pass would be "illegal" and "unsuccessful", Hague warned.