Business leaders who donated more than £400,000 to Northern Ireland's Democratic Unionist Party to campaign for Brexit are preparing to invest even more in opponents to Scottish independence should Nicola Sturgeon call a second referendum.

The shadowy donor group can today be revealed as a pro-union group comprising a string of business figures called The Constitutional Research Council (CRC).

It provided the resources for the DUP to pay for a full wrap-around advert in the free Metro newspaper in the days before the vote, even though the paper is not published in Northern Ireland. The money also went to other campaign material distributed across the country.

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It has now emerged the CRC, which was established amid disquiet about the way Better Together ran the pro-union campaign in the 2014 referendum, will bankroll credible unionist groups in any forthcoming Scottish independence referendum

A source close to CRC called for a “better class of politics” than in the last independence referendum and said there could be "even more" to spend than it did on the Brexit campaign.

The CRC said that research commissioned for the group showed that the often negative campaigning of the Better Together campaign “damaged the Union”.

Better Together boss Alistair Darling consistently defended the cross-party organisation’s approach, saying that to point out the downsides associated with an independent Scotland was highlighting the benefits of the Union.

The CRC backed Brexit in last year’s European Union referendum after coming to the conclusion that leaving the EU would be good for the Union and "bad for nationalism".

But it did not want to run its own Leave campaign, so it decided to donate to one.

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

But the DUP has been under increasing pressure for weeks to reveal the identity of its secret benefactor.

Campaigners have called for a change in the law to force Northern Irish parties to follow the same rules as those in the rest of the UK, amid accusations of “donor tourism”.

The elections watchdog, the Electoral Commission, will publish details of EU referendum campaign spending over £250,000 later today.

In the immediate aftermath of the Brexit result First Minister Nicola Sturgeon declared that a second independence vote was now "on the table" and a number of opinion polls suggested a spike in support for independence.

That rise in support appeared to fall off again in the months after the vote.

But a BMG poll for The Herald earlier this month suggested it was on the rise again after Theresa May's recent "hard Brexit" speech setting out her priorities for negotiations with Brussels.

The CRC says it has no ambition to be the official pro-Union campaign in any new independence referendum, but wants to donate to the right kind of message.

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Richard Cook, the chairman of the CRC and a former vice-chairman of the Scottish Conservatives, said: "In reality Better Together’s campaign at the last independence referendum suppressed the pro-Union vote due largely to it having adopted an overwhelmingly negative approach as to why Scotland shouldn’t be independent, rather than being evangelically for the Union and selling its benefits to Scotland, and Scotland’s benefits to the rest of the UK.

"If there is to be a second independence referendum the next pro-Union campaign needs to be a “better Better Together” talking about how Scotland benefits from its place in the Union, how we’re a progressive outward looking people and how there’s a positive future for the United Kingdom which makes every part of it better.”

He added that the pro-Union campaign also needed to adopt more modern campaign techniques, including harnessing social media.

Better Together, dubbed “Project Fear” by Nationalists, was dogged by claims of negativity almost from the start.

A notorious television advert designed to appeal to women led to accusations of patronising sexism.

There was also criticism of the role played by Conservative ministers, especially the then chancellor George Osborne’s decision to travel to Edinburgh to rule out a currency union with an independent Scotland.

He did not stay overnight, leading to claims that he was an English ‘day tripper’.