The DUP was last night dismissing criticism of its links to the businessman in line for a multi-million pound development at the Giant's Causeway.
Seymour Sweeney, a DUP member, learned on Monday that DUP Minister Arlene Foster is poised to grant planning permission for his visitor centre plans for Northern Ireland's top tourist attraction.
Ms Foster yesterday denied any knowledge of the developer's background and threatened to sue anyone who "impugns my integrity in this matter".
She said: "I do not know the applicant, have never met him, and know nothing about him."
It has also emerged that Mr Sweeney sold a north Antrim property some years ago to DUP politician Ian Paisley Jnr, who is now a junior Stormont Minister.
Mr Paisley is currently listed as a member of the businessman's management company for the cottage development site.
Mr Sweeney yesterday stressed that the property sale to the DUP Assemblyman had been at full market price, and said the management company is to be fully transferred to all the property purchasers.
Mr Paisley Jnr told the Belfast Telegraph last night that everything was " open and above board" and also accused opponents of being " small-minded".
He has been one of the leading champions of Mr Sweeney's bid to build a private sector visitor centre for the Giant's Causeway.
The developer's proposal has been strongly opposed by the National Trust, the charity that owns the Causeway stones.
On Monday, the Environment Minister announced that she was "minded" to approve a planning application from Mr Sweeney's company, Seaport Ltd, for a visitor complex.
Her party colleague, Enterprise Minister Nigel Dodds, then signalled that he would be pulling the plug on plans for a publicly-funded Causeway centre.
The choreographed move by the two Ministers was criticised by other parties, with Sinn Fein and SDLP MLAs later calling for clarification on Mr Sweeney's DUP connections.
These include membership of the party and the fact that he signed election nomination papers for DUP Moyle councillor David McAllister.
In a strongly-worded statement yesterday, Ms Foster said she had made her announcement on the Causeway centre application "based on the correct and proper procedures which govern my conduct as Minister".
She continued: "Today there have been reports, in some sections of the media, which might have implied to some that I may have allowed other extraneous factors, ie the party political membership of the applicant, to affect my views on this application.
"I totally reject any such implications. I regard them as baseless and scurrilous. A minister, when making decisions, cannot and should not investigate party affiliations of applicants, and I did not do so in this case. I do not know the applicant, have never met him, and know nothing about him.
"If anyone impugns my integrity in this matter I will be seeking legal advice and will act accordingly."
Mr Sweeney yesterday also stated that he had never met Mrs Foster or Mr Dodds.
A statement issued through his PR company further said: "Mr Seymour Sweeney, Managing Director, Seaport (NI) Ltd, can confirm that he is a member of the DUP and like all members pays his annual subscription at the usual rates.
"Neither he nor any of his companies have now, or ever, been a donor to the Party. Mr Sweeney has no other links to the DUP either personally or through family connections."
It also stated: "He can confirm that many years ago at one of Seaport's Developments, namely Ballyallaght Farmyard Cottages outside Bushmills, a property was purchased by Ian Paisley Jnr in the normal way through sales agents and solicitors.
"Ian Paisley Jnr paid the full market sales price which was more expensive than the house neighbouring his property by virtue of the fact that the house he bought was slightly larger."
Mr Paisley's nomination papers for this year's Assembly elections gave 1 Ballyallaght Farm Cottages as his address.
Latest returns at the Government's Companies Register list Mr Paisley Jnr as one of a small number of company members for Ballyallaght Management Ltd - each holding one share each.
Mr Sweeney is listed as a director. He told the Belfast Telegraph last night that the management firm's role involves maintenance of exterior lighting, grass, roads and sewerage at the Bushmills development.
He said each property purchaser is made a shareholder and member and the company would be fully transferred to all the owners once the development is completed.
Mr Paisley Jnr last night said: "I have no questions to answer on this. I believe it's all open and above board. The property was bought a number of years ago through normal channels."
The DUP junior minister also said the Ballyallaght management company was a normal arrangement, that will deal with communal matters.
He said people questioning Mr Sweeney's DUP links were being " small-minded" and "mischievous".
Speaking in the Assembly earlier yesterday, Mr Paisley said: "What we are having is an argument from the benches opposite to discriminate against an applicant, not to look at the salient points contained in the application.
"Is this the right way forward for Northern Ireland in terms of delivering the world class tourism centre of excellence or is it not?"