Arron Banks, millionaire backer of Brexit campaign Leave.EU. Photo: Ben Birchall/PA Wire

Arron Banks, the maverick tycoon who funded the Leave.EU campaign with £5.6 million of his own money, made the allegation in a book which recounts an extraordinarily divisive campaign – both between the two sides, and within each camp.

Mr Banks, who has also poured millions of pounds into Ukip, devotes several pages of his book, The Bad Boys of Brexit, to his dealings with the DUP and claimed to be horrified at the party’s actions.

But, in a very public falling out within the pro-Brexit camp, the DUP last night rejected the claims as “entirely false” and countered by alleging that Mr Banks had offered the party money to support his group.

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Mr Banks – who has been described by the Daily Mail as a “roguish, larger-than-life multi-millionaire” and “instinctively confrontational” figure who “fell out with virtually everyone” during the campaign – is scathing about the DUP’s approach.

In a diary entry for 12 February 2016, Mr Banks records: “The DUP is demanding cold, hard cash in exchange for its support! Thirty grand a month, to be precise. I know Northern Irish politics is dirty, but this is crazy. It all came about because Farage is mates with Ian Paisley Jr...”

Mr Banks said that “we already know the DUP are Outers. The question is whether they endorse us or Vote Leave..

“Nigel [Farage] suggested I talk to a guy called Chris Montgomery, the party’s chief of staff at Westminster, and see if we could sign them up.”

Mr Montgomery is a Londonderry-born former journalist who was well known in Conservative and unionist circles in London prior to working for the DUP at Westminster in recent years.

Mr Banks said that even before he met the DUP representative he was distinctly unimpressed by Mr Montgomery’s choice of language in a text message which he referred to an estate agent (Mr Montgomery was house-hunting at the time) as a “slimy turd”.

Mr Banks wrote: “When he finally turned up, he shamelessly announced that if the DUP were to come on board, they were going to need thirty grand a month for four months. I told him he must be bloody well joking if he thought we were going to hand over that amount of money (allegedly to ‘spend on the campaign’).

“[I] made it clear that’s not the way we operate and that we want the support of parties that believe in what they’re doing, not people who can be bought.”

In the end, Mr Montgomery went on to become a member of the rival Vote Leave’s board, along with DUP deputy leader Nigel Dodds.

In a statement to the News Letter last night, the DUP said: “The claims in Mr Banks’ book are entirely false. Arron Banks and Nigel Farage approached the DUP to ask the party to support first Leave.EU, then Grassroots Out’s bid to be designated lead campaign in the EU Referendum, rather than endorse Vote Leave’s bid.

“Mr Banks offered the party financial support if it changed which bidder it backed.

“The party neither accepted money from Arron Banks nor changed its mind.

“As the Referendum result demonstrated, Vote Leave was the right choice. Any other choice as designated lead campaigner for Leave would inevitably have seen the Referendum lost.”

Elsewhere in the book, Mr Banks suggests that the DUP eventually got around £50,000 a month out of Vote Leave, although the claim is less detailed that his initial allegation about the meeting with Mr Montgomery. Last night Matthew Elliott, chief executive of Vote Leave, told the News Letter that that it was “100% not true that Vote Leave gave any money to the DUP”.

When asked if there was any other means, outside of the campaign itself, by which money was channelled to the DUP, he said: “No.”

Mr Elliott was also asked about the highly unusual - and vastly expensive - wraparound national newspaper adverts which the DUP took out in the final days of the campaign.

The party paid for four pages of ‘Take Back Control’ adverts in Metro, a paper which is not even circulated in Northern Ireland. When asked if that was funded through Vote Leave or whether he knew about it, Mr Elliott said that the averts “were taken out by the DUP” and said that his organisation was “not at all involved in it”.

After meeting, relations deteriorated rapidly

Arron Banks’ diary entry in which the meeting with the DUP is recorded is dated 12 February.

At that point there was an intense battle between his insurgent Leave.EU campaign and the more mainstream Vote Leave group, both of which were attempting to convince the Electoral Commission that they were the best placed to fight the campaign and should be designated as the official campaign.

At the time, there was public speculation about the DUP’s actions, alhtough there were no suggestions that money was part of the Although the party had historically been vehemently anti-EU, when it became clear that a referendum would be held, the DUP did not immediately come out and say it would be definitely campaign to quit the EU (as the SDLP did in immediately saying that it would back Remain).

Instead, the DUP insisted on waiting for the outcome of the Prime Minsiter’s renegotiated terms of EU membership before deciding to come out for Brexit.

And, just four days after Mr Banks’ diary entry the party was forced to publicly deny that it was backing Leave.EU. Grassroots Out - which was linked to Leave.EU – had claimed that DUP MP Sammy Wilson endorsed it. Mr Wilson had spoken at a Leave.EU event in England. However, the DUP released a statement denying that it had decided to back the group, describing its claim as “utter codswallop”. In evidence of obvious strains with Mr Banks’ outfit, a DUP spokesman was quoted at the time as telling the Politics Home website: What’s most disappointing for us is not that they sent out a wrong press release, it’s that having agreed to change it they have gone back on their on their word.