Politicians across Britain and Ireland have called for the Democratic Unionist Party to reveal the source of a £435,000 (€500,000) donation it received in the run-up to the UK Brexit referendum last year.

The DUP, which backed the proposal for the United Kingdom to leave the European Union, spent heavily during the campaign, including £282,000 (€325,000) on a newspaper advertisement and £32,750 (€37,500) with a data-analysis company linked to Donald Trump. It spent only about £10,000 in Northern Ireland.

The DUP has said that the £435,000 came the Constitutional Research Council, a group whose only publicly declared member is its chairman, Richard Cook, a former Conservative general-election candidate who lives near Glasgow and has business links with Saudi Arabian intelligence services. The council does not publish accounts and has refused to name its funders. Northern Irish election law allows political donations to be kept secret.

Politicians and campaigners from across the spectrum told The Irish Times that the DUP should reveal who funds the council. The call comes as the DUP is set to support Theresa May’s minority Conservative government.

The Conservative MP Tom Tugendhat said: “Democracy depends on openness. That’s why we have transparency legislation on campaign donations.”

The Scottish National Party said failure by the DUP to reveal the names of the donors would “cast a dark shadow over Theresa May’s government” and its negotiations with the EU.

With the DUP now set to be the power behind Theresa May’s throne, clarity over this donation is needed as a matter of urgency

“With the DUP now set to be the power behind Theresa May’s throne, clarity over this donation is needed as a matter of urgency,” the SNP MP Pete Wishart said. “It’s deeply concerning that we still don’t know the source of such a significant amount of cash that bankrolled the Brexit campaign.”

The controversial £425,000 donation is widely considered to be the largest sum ever given to a Northern Irish political party. During the 2016 Stormont elections the DUP spent less than £90,000.

The £282,000 advertisement was a four-page wrap-around for the Metro newspaper, which is not available in Northern Ireland. The £32,750 spent on data analysis went to AggregateIQ, to target voters on social media. The company has been linked to Cambridge Analytica, a firm funded by the billionaire Robert Mercer, who heavily backed Brexit and Mr Trump’s successful presidential campaign.

Reports in the UK suggest the Democratic Unionists want Nigel Farage, the former Ukip leader, and the millionaire Brexit funder Arron Banks involved in the Brexit talks. Mr Banks has denied that he was involved in DUP’s Brexit donation but has said that the DUP asked him for money to support his Leave.EU campaign.

In February this year, during a televised debate ahead of snap Stormont elections, the DUP leader, Arlene Foster, said the donation came from “an organisation in England that wants to see the Union kept” but refused to give any more information.

Later that month the DUP said the money came from the Constitutional Research Council. Richard Cook’s business associates include Prince Nawwaf bin Abdul Aziz Al Saud, a former director general of the Saudi Arabian intelligence agency. In 2013 Mr Cook set up a management company focused on the Middle East. His codirector was a Danish man named Peter Haestrup, who was named by India’s Central Bureau of Investigation in connection with a notorious 1995 arms drop. Mr Haestrup was never charged with any wrongdoing.

The UK Electoral Commission records donations to Northern Irish parties, but these are not made public because of security concerns dating back to the Troubles. This ban, called the Prescribed Period, was due to last only until October 2010, but that date has been repeatedly extended.

Now that the DUP are wielding influence over the UK government, continued secrecy around their finances is completely untenable

The leader of the Alliance Party of Northern Ireland, Naomi Long, called for the secretary of state to end the prescribed period “without further delay” and for the DUP to reveal details of the party’s Brexit funders. “The situation in NI, where parties can receive huge donations yet not disclose them to the public, ought to have ended many years ago. However, now that the DUP are wielding influence over the UK government, continued secrecy around their finances is completely untenable,” Ms Long said.

The former UK Green Party leader Natalie Bennett said: “Given the apparently key role the DUP are going to play in deciding the future of Britain, we must have clarity about where this money came from and what steps the party took to check its source.”

The DUP did not spend all of the £435,000 Brexit donation. At the end of the campaign £9,000 (€10,300) was transferred into normal party funds.

The DUP declined to comment on this story.