UK sanctions law bans firms from dealing with funds from institutions blacklisted by the EU or the UN either directly or indirectly, or providing any resources which could help them make money. Kushner, the sanctions lawyer at W Legal, said that although the payment codes listed in the Almanac are technically publicly available, it would be virtually impossible for banks to do business without access to the database, because “by the time they had got the information independently it would be too late to process the payment".

John Mann MP, a prominent member of the powerful House of Commons Treasury select committee, said it was “no defence” for RBI to claim that the law on dealing with blacklisted banks was unclear. “Any company trading in Syria and Iran knows what it is doing, and investing in these countries despite the sanctions is a disgrace,” he said. “Hiding behind the supposed ‘complexity’ of sanctions is no excuse.”

The sanctions screening service RBI offers its own clients through Accuity is designed to prevent banks from accidentally dealing with blacklisted entities. “No one should know sanction laws better than someone publishing sanctions screening information and data,” said Kushner.

The economic crime division of the City of London police has confirmed it is now examining evidence gathered by the Middle East salesman after it was passed information by investigators at the Serious Fraud Office in February this year. The National Crime Agency’s economic crime command and the criminal investigations division at HM Revenue and Customs have also been reviewing evidence of possible sanctions busting by RBI.

However, no action has yet been taken since the Middle East salesman first blew the whistle to the authorities in January last year, and RBI said that following an internal investigation, there was “no basis” for anyone at the firm to be sacked or disciplined for their role in fixing deals with sanctioned banks.

A spokesperson for the Foreign Office said: “The UK is committed to ensuring that sanctions are adhered to and properly enforced. A range of restrictive measures are in place on both Iran and Syria … It is the responsibility of economic operators in the UK to ensure they are compliant with sanctions. Failure to comply with sanctions can lead to criminal prosecution.”

Anyone found guilty of breaching EU or UN sanctions in the UK is liable to receive a fine or imprisonment of up to two years, and businesses can also face punishment. The penalties are much tougher in the US, where those convicted of busting sanctions can be imprisoned for up to 20 years and receive fines of as much as $1 million per violation.

As well as dealing with banks that were blacklisted by the EU and the UN, RBI provided services to a further 11 banks in countries sanctioned by the US Treasury over links to terrorism and nuclear weapons, the documents show.

While EU and UN sanctions tend to apply only to specifically blacklisted entities, the US regime uses more sweeping measures to ban trade with a wide range of organisations in sanctioned countries such as Syria, North Korea, and Iran.

RBI said the Bankers’ Almanac is not subject to US sanctions, because it is registered in the UK. But in 2011 its parent company, Reed Elsevier, bought the US-based financial information and sanctions-screening firm Accuity and RBI began selling the database using that brand. Like all US-based entities and individuals, Accuity is required to comply with the country’s sanctions regime.

The documents show RBI continued providing the Bankers’ Almanac to 13 entities under US sanctions in Syria, Iran, and Iraq after it acquired Accuity and began using the brand in November 2011.

A spokesperson for the company said that despite the acquisition, "BankersAlmanac has remained a UK entity, subject to EU and UK sanctions law." He insisted: "At no point did Accuity make any sales to any entity sanctioned by the US."

"Nonetheless," he said, "a decision was made in 2012 to stop BankersAlmanac sales to entities subject to US sanctions law, even when those sales would be legal." Despite this, documents seen by BuzzFeed News show that some of RBI's contracts with banks blacklisted by the US Treasury rolled on until the salesman blew the whistle in 2013.

A source at RBI said they had "slipped through".

RBI’s insistence that it was under no moral or legal obligation to obey US sanctions comes despite the fact that its parent company has been deeply entwined with American politics for many years. Reed Elsevier has made campaign donations totalling more than than $2 million to the Democrats and the Republicans since 1993, including $55,000 to Barack Obama. Mark Kelsey, chief executive of RBI, was invited to speak alongside the president at a White House cybersecurity conference in February this year.

The documents reveal that RBI has contracts to sell the Bankers’ Almanac to the US Treasury – the very government department in charge of the country’s sanctions regime – as well as the Department of Defense.

The Middle East salesman said it was plainly wrong for RBI to have done business with banks blacklisted by the US.

“These are banks that have been implicated in terrorism, in money laundering, in nuclear proliferation,” he said. “Banks who pose a huge risk to international peace and security. It’s very clear-cut: If you have a blacklisted bank, you shouldn’t be doing business with them.”