PRO-EU campaigners are set to launch a court bid to expose what they regard as a “whitewashed investigation” by the elections watchdog into a £435,000 pro-Brexit donation to the Democratic Unionist Party during the 2016 EU referendum campaign.

The Good Law Project, founded by QC Jolyon Maugham, and Labour’s Ben Bradshaw, the former Culture Secretary, are behind the planned legal move at the High Court in London, which claims the Electoral Commission acted “without due diligence” in respect of its probe into the Constitutional Research Council’s donation to the DUP and its failure to investigate fully the “secretive” CRC, headed by Richard Cook, a former Vice-Chairman of the Scottish Conservatives.

If the campaigners succeed in a judicial review of the case, then they will ask the Commission to “forfeit” the CRC’s donation to the DUP.

Arlene Foster, its leader, has insisted the £435,000 gift was properly accounted for. Her DUP colleague Sir Jeffrey Donaldson has explained that the CRC money was spent on pro-Brexit advertising throughout the UK, noting how the referendum campaign was "not a Northern Ireland referendum but a UK-wide referendum".

Because of the Troubles, donations to political parties in Northern Ireland are confidential amid fears of reprisals if donors are identified.

The £435,000 donation to the DUP has been raised at Westminster by the SNP, which regard it as party of the Tories’ “dark money”.

In March, Ian Blackford, the party’s Westminster leader, asked Theresa May about what he called was a “shady business”. The Prime Minister made clear that it was for the Commission to investigate any alleged breaches of campaign spending.

Mr Cook is a prominent figure in Scottish Tory circles, having stood in several elections, most recently in 2010 in the Westminster seat of East Renfrewshire. He chairs the CRC, the group of pro-Union businessmen, about which little is known.

It was established amid disquiet about the way Better Together ran the pro-Union campaign in the 2014 referendum and has pledged to bankroll any credible Unionist groups in any future Scottish independence referendum.

The CRC backed Brexit in the 2016 referendum after concluding leaving the EU would be good for the Union and "bad for Nationalism".