Britain, Germany and France joined forces tonight in a call to Iran to abandon "a path that threatens the peace and security of us all".
A statement in the names of the Prime Minister, chancellor Angela Merkel of Germany and French president Nicolas Sarkozy followed agreement by EU foreign ministers meeting in Brussels on a ban on Iranian crude oil exports.
The three leaders described the sanctions package - designed to seriously hinder the regime's economic muscle - as "unprecedented" and said: "Our message is clear: we have no quarrel with the Iranian people, but the Iranian leadership has failed to restore international confidence in the exclusively peaceful nature of its nuclear programme."
The statement continued: "We will not accept Iran acquiring a nuclear weapon. Iran has so far had no regard for its international obligations and is already exporting and threatening violence around its region."
The warning came as an American aircraft carrier, joined by a British Royal Navy frigate and a French warship, went through the Strait of Hormuz, which Iran has threatened to block in response to the oil ban.
With tensions in the region mounting, Britain, Germany and France reinforced a statement from the Brussels talks by declaring: "We call on Iran's leadership immediately to suspend its sensitive nuclear activities and abide fully by its international obligations.
"The door is open to Iran to engage in serious and meaningful negotiations about its nuclear programme. Until Iran comes to the table, we will be united behind strong measures to undermine the regime's ability to fund its nuclear programme, and to demonstrate the cost of a path that threatens the peace and security of us all."
Earlier Foreign Secretary William Hague emerged from the talks in Brussels to welcome the oil sanctions, which had full British backing.
"This shows the resolve of the European Union on this issue," he said.
"It is absolutely right to do this when Iran is continuing to breach United Nations resolutions and refusing to come to meaningful negotiations on its nuclear programme."
Mr Hague said he hoped the EU's firm action would influence the attitude of many other countries.
"The EU is taking a very strong lead in world affairs and we strongly welcome this."
The Foreign Secretary said he expected the sanctions to be "very effective" in opening Iran up to engaging in "meaningful" talks.
Between them, EU member states buy almost 600,000 barrels a day of Iran's crude oil exports, which total 2.6 million barrels a day worldwide.
That makes the EU the largest single market for Iranian oil, with Greece relying on Iran for one third of its oil imports, while 13% of Italian consumption comes from Iran, compared with almost 10% in Spain.
The problem is that the sanctions are therefore likely to hit three of the EU's struggling economies hard, thanks to preferential payment deals with Iran which will make it hard to find suppliers elsewhere without adding to their debts.
The sanctions deal involves an immediate ban on new oil contracts with Iran and a phasing out of existing contracts, which must end by July 1 at the latest.
A ban on exports of gold from Iran will also apply, with more travel bans and financial asset blocks on members of the Iranian regime.
Conservative MEP Charles Tannock, the party's foreign affairs spokesman in the European Parliament, said: "Iran's nuclear military ambitions and its government's refusal to engage with the rest of the world on this issue constitute a major threat to global stability and a potential risk to peace in the Middle East.
"The measures agreed by the EU, in denying Iran some of its oil revenues and by making it harder for the leadership to move money around the world, will put pressure on Tehran to come back to the negotiating table."