Britain on Sunday urged Iran to cooperate with the international community after the EU slapped an oil embargo on Tehran designed to turn up the heat on the regime.
Foreign Secretary William Hague said the sanctions showed the European Union's determination to get tough with Iran.
"Unprecedented oil sanctions on Iran have come into force," Hague told BBC television.
"These are the toughest measures the EU has adopted against Iran to date. They signal our clear determination to intensify the peaceful diplomatic pressure on the Iranian government."
The EU embargo on Iranian oil went into effect on Sunday, provoking anger in Tehran which says the measure will hurt talks with world powers over its sensitive nuclear activities.
Britain is one of the six countries dealing with Iran over its nuclear programme.
The group comprises the five permanent members of the United Nations Security Council -- Britain, China, France, Russia and the United States -- plus Germany.
Iran must make a choice in the coming weeks, Hague said.
"It can continue to obfuscate and avoid the critical issues, incurring tough sanctions and increasing international isolation.
"Or it can begin to cooperate seriously by discussing the steps it is prepared to take on its nuclear programme, and seizing the opportunity to secure a more prosperous and peaceful future for the Iranian people.
"We urge the Iranian government to reflect seriously on its position and return to the negotiating table ready and willing to make diplomacy work."
The 27 EU nations account for 20 percent of Iran's oil exports -- which bring in 80 percent of the country's foreign reserves -- while 85 percent of marine insurance is provided by EU firms.
"We are seeking a diplomatic, negotiated settlement with Iran. We call on Iran to take concrete action to address international concerns about its nuclear programme," Hague said.
"It is in the power of the Iranian leadership to end Iran's current isolation, but unless they change course the pressure will only increase."
Talks -- at expert level -- between Iran and the six world powers are to be held on Tuesday in Istanbul.