Conservative Party leader David Cameron and foreign affairs spokesman William Hague said there was a leadership vacuum in government when Britain should be pushing its international partners to renew efforts to tackle Tehran.
British Prime Minister Tony Blair announced earlier this month that he would step down on June 27.
Chancellor of the Exchequer Gordon Brown has since accrued enough votes from Labour politicians to make a leadership contest unnecessary, but he will not take over as prime minister any earlier. Brown has said he believes the Iran sanctions are working and has not yet elaborated on ideas for future action.
''Every week, every month that goes by brings Iran closer to possessing a nuclear weapon. We need to recognise that the current response to Iran's nuclear programme is not yet working ... that is why we need a shift in policy -- as rapidly as possible,'' Cameron told a news conference.
The International Atomic Energy Agency is due to give its assessment on May 24 of Iran's compliance with previous UN resolutions requiring it to halt uranium enrichment.
Iran has refused to bow to Western demands to stop its nuclear programme and insists it aims only to yield electricity. The West suspects it of trying to build nuclear weapons.
PENALTIES ''It is from this week onwards that nations across the world have to decide what they're going to be doing about it. It would be better for the country on this issue if we had a prime minister who was going to be in office for more than the next six weeks,'' Hague said.
Cameron said Britain should use its ties to the European Union and the United States to press the former for harsher penalties and the latter for more enticing incentives.
EU states must be persuaded to clamp down on Iranian banks' access to the European financial system and begin targeted action to restrict European investment in Iranian oil and gas fields, he said.
A third round of sanctions at the United Nations must be much tougher, closing any loopholes which allow countries to impose their own interpretations of measures such as restricting travel, arms sales, financial assistance and training in nuclear fields to Iranians, he said.
Britain should also convince Washington to discuss with Tehran the framework of any subsequent negotiations once Iran suspends its nuclear programme, Cameron added.
Responding to criticism that his suggestions were more wish-list than practical, the Conservative leader said: ''You never know unless you try. This is the most urgent problem facing the region and potentially our world and so it's incumbent on us to push this forward.'' Asked whether he would support military action against Iran, Cameron said it would be a mistake to rule it out.
''We are trying to make sure that we do everything to avoid that happening. It would be a calamity if they got a nuclear weapon; military action would be calamitous.'' Reuters SS GC1828