It is a strange tale involving Theresa May’s new “friends” in the Democratic Unionist Party, the Brexit referendum campaign and a mysterious donation of £425,000.

And, while those involved have refused to reveal more than a few details, there has been allegedly “fanciful” talk of “an arms drop in India” and the involvement of Russian President Vladimir Putin and Saudi intelligence.

The story began with a four-page, wrap-around cover advert in the Metro newspaper promoting the Leave campaign ahead of the Brexit referendum in June 2016, an extraordinary move that cost some £282,000.

This turned out to have been paid for by the DUP, which was odd as the party only stands in Northern Ireland where the Metro is not published.

Under Northern Ireland’s electoral laws, political donations do not need to be publicly declared, a legacy from the region’s Troubles, when funding a political party might make someone a target for terrorists.

However in February, after some political pressure over the issue, the DUP MP Sir Jeffrey Donaldson revealed that the adverts had been paid using a £425,000 donation from a group called the Constitutional Research Council (CRC), chaired by former Scottish Conservative vice-chairman and Glasgow-based businessman, Richard Cook.

This sparked further questions about the ultimate source of the money. Mr Cook told The Sunday Times that the CRC had three other members, but declined to identify them.

He did decide to try to dampen down some of the speculation about who had been involved.

“There have been rumours that the DUP got this money from Vladimir Putin, dark shadowy groups, then Saudi intelligence and even from beneficiaries of an arms drop in India. It is all fanciful nonsense,” he told the paper.

He said the group had not received foreign funding and added that he was in touch with business people prepared to donate six-figure sums to help the Union campaign in the event of a second Scottish independence referendum.

The Saudi intelligence link was suggested by Adam Ramsay, co-editor of the openDemocracyUK website.

By examining documents held by Companies House, he discovered Mr Cook and the former director-general of the Saudi intelligence service, the late Prince Nawwaf bin Abdul Aziz, had founded a company called Five Star Investment Management in 2013, which was dissolved the following year.

Mr Ramsay said he had no evidence that the £425,000 had come from a Saudi source.

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But he added: “Saudi Arabia is a country flush with a huge amount of cash and I don’t think it’s crazy to think they would a, have an interest in a major event like Brexit and b, try to influence it in some way.”

Saudi economists, he noted, had argued that the referendum result was good for the country because of the subsequent fall in the value of the pound.

The routing of the money via the DUP effectively maintained the anonymity of the donor.

The DUP’s press office could not be reached for comment on Friday. The Independent contacted Sir Jeffrey Donaldson’s constituency office and was told attempts would be made to pass a message to him.

Kellie Armstrong, a member of the Stormont Assembly representing Strangford, said her party, the non-sectarian Alliance, and the Greens were the only ones in Northern Ireland to publish their donations.

She said, given it was “19 years since the Good Friday Agreement was signed”, the time had come to be open about the sources of political funding.

“I’ve challenged the DUP as to why they do not publish their donations,” she said.

“A number [of other parties] have said they would go forward and publish their donations. The DUP has said that, but they haven’t done it yet.”

She said it was strange for the DUP to pay for political adverts people who might vote for them would never see.