Britain's export credit agency is planning to review Iran's creditworthiness, it said on Thursday, one of the first steps to normalising trade between the two countries after this week's nuclear deal between world powers and Tehran.

With just under 80 million people and annual output of some $400 billion, Iran is set to be the biggest economy to rejoin the global trading and financial system since the break-up of the Soviet Union over two decades ago.

But many sanctions on the country look set to remain for months. US and European banking restrictions, for example, will be lifted only when the International Atomic Energy Agency has verified that Iran is keeping to its side of the nuclear bargain.

"We will be initiating a review of Iran to assess creditworthiness, in light of the new agreement, and the expected positive effect on the Iranian economy," UK Export Finance (UKEF), which provides guarantees to banks and insurance to support British exporters, told Reuters.

"UKEF has outstanding claims on Iran and we would expect these arrears to be regularised to a large degree before we would be in a position to restore full cover," it added.

A spokeswoman said there was no specific timeframe for the review and declined to provide further details on the arrears.

Prior to the sanctions, Britain was a major trading partner with Iran.

London listed oil major Shell said this week it was interested in doing business in Iran, one of the world's top oil producers.

Parts of Britain's lucrative financial services industry -- which includes the Lloyd's of London insurance market -- will also be vying for business.

But Britain faces competition.

Italy's export credit agency SACE said on Tuesday a gradual lifting of sanctions could lift Italian exports to Iran by around €3 billion ($3.3 billion) in the next four years.

British banks will also be cautious about doing business in Iran given past experiences, industry sources said.

UK banks are pulling out of controversial businesses and slimming international operations in response to recent scandals. HSBC was fined $1.9 billion in 2012 by US regulators for violations including doing business with Iran, while Standard Chartered in 2012 paid US authorities $667 million for violating US sanctions and then a further $300 million after more compliance shortcomings were uncovered.

Britain said in an updated trading note on Tuesday that: "If Iran adheres to its commitments and sanctions are rolled back, then the government will help the business and financial sector take advantage of the opportunities that arise, and promote trade and investment between our two countries."

Foreign minister Philip Hammond added on Wednesday that Britain remained committed to re-opening its embassy in Iran.

Nigel Kushner, a director with the British Iranian Chamber of Commerce association, said trade prospects would depend on Iran complying with a "multitude of obligations".

"The reality is that there will be no tangible change in the EU or US sanctions regime for at least six to nine months at best," said Kushner, a leading London-based sanctions lawyer. - Reuters