Ian Paisley Jnr directly lobbied Secretary of State Peter Hain two years ago in support of private sector involvement at the Giant's Causeway, the Belfast Telegraph has learned.
And the MLA's representations also expressed hostility to the public sector visitor centre project being developed by Mr Hain's Ministers.
Mr Paisley Jnr is now facing calls to disclose the full extent of his lobbying for north Antrim developer Seymour Sweeney, the DUP member behind controversial plans for a commercial Causeway centre.
The politician has not made any response to the challenges, and has maintained that he has every right to speak up for a businessman in his north Antrim constituency.
It has emerged that Mr Paisley Jnr and DUP Assembly colleague Mervyn Storey wrote a lengthy joint letter to Mr Hain in June, 2005 highlighting a number of issues of concern.
They argued that tourism at the Giant's Causeway was "not being marketed or exploited to its full potential".
The letter continued: "We believe that tourism can only succeed where the experts are involved and that most certainly includes the private sector. Without their expertise tourism will not grow significantly."
Mr Paisley Jnr has been a long- term supporter of Mr Sweeney's visitor centre plan for the Giant's Causeway.
The letter to Mr Hain further claimed that alternative Government plans for a new Causeway building were "a time marking exercise".
And they said the blueprint being followed in this process was "inadequate and constrictive" and would result in a centre that would be "too small to meet tourist needs".
The Government process - now shelved by DUP Enterprise Minister Nigel Dodds - was based on development restrictions sought by UNESCO, the body responsible for the Causeway's World Heritage Site status.
Mr Paisley Jnr has also backed Mr Sweeney on other issues in recent years.
He supported him over a Causeway pub development called the Nook that was unsuccessfully opposed by the National Trust; gave evidence for the developer at a 2005 planning appeal on a Bushmills housing development; and lobbied for a drift net licence for a north coast fisherman whose crew included Mr Sweeney.
The DUP politician also publicly backed a successful legal challenge by the businessman against the Northern Area Plan, a DoE development framework.
The letter sent by Mr Paisley Jnr to the Secretary of State is listed as part of his representations to the Northern Area Plan consultation process.
It also called for the Plan to be revised to allow greater housing development in north Antrim and demanded a "sensible planning strategy" that was "not the voice of a limited so-called environmental lobby".
Mr Paisley Jnr and his DUP leader father made a joint submission in July, 2005 to the Northern Plan process.
It objected strongly to proposals for an increase in green belt zoning and pressed for more housing for Moyle, Bushmills and Ballymoney.