Opposition lawmaker Ben Bradshaw last week urged the government to look into reports by advocacy group Open Democracy suggesting that the origin of some Brexit campaign funds was unclear.

At a regular briefing with reporters, May’s spokesman was asked if the prime minister was concerned about the reports.

“I am not aware of those concerns,” he said.

“More broadly, as we’ve always said, the UK democratic system is amongst one of the most secure in the world and will continue to be so.”

Open Democracy, which describes itself as an independent media platform aiming to challenge power and encourage debate, had raised questions about a large donation to Northern Ireland’s Democratic Unionist Party (DUP) which is now propping up Britain’s minority Conservative government.

Open Democracy said the origin of the money, which was spent on campaigning for Brexit, was unknown.

The DUP, who was earlier reported to have said the funds came from “an organisation in England that wants to see the union (between the UK’s constituent parts) kept”, did not immediately respond to requests for comment.

Open Democracy also published a report into the finances of Arron Banks, a businessman who was one of the main financial backers of the Brexit cause. It called for greater clarity on the source of Banks’s fortune.

In a statement, Banks dismissed Open Democracy’s report without addressing it in detail. He suggested Bradshaw’s comments came from his inability to come to terms with the Brexit camp’s victory in the 2016 referendum.

The Electoral Commission, which regulates political finance in Britain, said in April it was investigating campaign spending by pro-Brexit organisation Leave.EU, without giving details.

A spokeswoman for the Electoral Commission said on Monday that investigation was still going on and it would not provide any further information until it was complete.

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