It has emerged that Ian Paisley Jnr lobbied in support of a Bushmills housing plan tabled by Mr Sweeney - and then bought one of the properties himself a few years later.

The MLA has denied any conflict of interest and said he was not planning a purchase at the time of his lobbying.

The Paisleys are meanwhile being dogged by further controversy over claims that Mr Sweeney's Causeway centre scheme had the official blessing of world heritage body Unesco.

The developer yesterday confirmed there has never been any such approval and retracted a past claim he made.

As the Telegraph revealed earlier this month, the DUP leader asserted in a 2003 letter to a public body that Mr Sweeney's project had Unesco backing.

The letter, on House of Commons notepaper, lambasted the Heritage Lottery Fund for turning down a £1.5m grant application from the businessman.

Mr Paisley has not responded to Press inquiries about his angry correspondence, while his MLA son has claimed there was "general support " from Unesco.

Speaking at a Press conference yesterday, Mr Sweeney said: "It is very important to clear this up. I have never said that we had Unesco approval for the scheme."

After the conference, the Belfast Telegraph showed Mr Sweeney a statement issued by his company Seaport in May 2002. It said: "At a meeting with senior Unesco personnel in Paris in September 2001, Seaport Investment representatives received wholehearted approval for its proposal for a world class visitor centre which was submitted to Moyle District Council the following month."

Mr Sweeney yesterday said: "You can put that to me and if I said that, that's fine, but it's not correct. I'm happy to retract that."

SDLP MLA John Dallat has made a complaint to House of Commons Standards Commissioner about Mr Paisley's 2003 letter. He today said: "It is surely time for the claims about Unesco approval to be publicly retracted by the First Minister and for an apology to be issued to the Heritage Lottery Fund.

"I would also reiterate my call to Ian Paisley Jnr to make full disclosure of the extent of his lobbying for Seymour Sweeney."

In a BBC Spotlight interview broadcast last night, Mr Paisley Jnr confirmed he lobbied Planning Service in support of Mr Sweeney's Ballyallaght Farm Cottages development near Bushmills. The MLA bought one of these properties in 2004.

Denying that this represented a potential conflict of interest, he said he didn't know he was going to purchase one at the time.

"Whenever the houses came on the market I bought one. And I bought it at the full market value and I wasn't advanced in any way or advantaged in any way by supporting the application," he added.

Mr Paisley has blamed an " administrative hiccup" for the fact his Ballyallaght house is still registered at Land Registry as owned by Mr Sweeney's wife, Carol. He said the house is owned and paid for by him, and said his solicitor has addressed the Land Registry issue.

He also claimed he was the victim of politically motivated "slurs" and "innuendo".

Mr Sweeney has also told this newspaper he could not rule out tabling proposals for further commercial development in the vicinity of his proposed Causeway centre, such as a new hotel.