And speculation that the subject was even raised at last year's pre-devolution talks in St Andrews has been heightened by the content of an internal Government document obtained by this newspaper.

Mr Paisley jnr has been a long-standing supporter of plans by fellow DUP member Seymour Sweeney for a commercial Causeway centre development.

Details of representations made by the politician have slowly emerged in the wake of a preliminary announcement by DUP Environment Minister Arlene Foster in favour of the businessman's scheme.

Mrs Foster has stressed that she was not lobbied by anyone over the Causeway proposals and that she had been unaware that Mr Sweeney was a DUP member.

Supporters of Mr Paisley jnr - a junior Stormont Minister - have defended his right to speak up for a businessman in his north Antrim constituency.

But the DUP MLA is still being challenged to spell out the extent of his lobbying in support of the developer.

The Belfast Telegraph understands from Government sources that he made personal representations to Direct Rule Environment Minister David Cairns on Mr Sweeney's planning application for a visitors' centre at the Causeway.

Mr Paisley jnr is also believed to have raised the subject with Secretary of State Peter Hain's senior team.

There have been suggestions from other political parties that Mr Sweeney's scheme was also raised in and around the St Andrews talks in October 2006 on the restoration of devolution - and that it was part of a "shopping list " that DUP negotiators wanted the NIO to address.

That speculation will be fuelled by the emergence of a high-level Government email dated October 24, 2006.

It was sent by a senior Department of Enterprise official to a civil service colleague in the Department of the Environment's Planning Service.

The memo related to the planning framework under which a decision would be made on Mr Sweeney's visitors' centre application. It asked for confirmation that these ground rules would only permit replacement visitor facilities and not an additional visitors' centre.

The memo added: "Grateful if you would come back to me by cop (close of play) today as this is required for ministerial briefing flowing out of St Andrews."

The email sent by the Planning Service in reply stated that a replacement to the existing visitor facility would be allowed, subject to its meeting planning policy requirements.

It also stated that, "as there is an existing visitor centre, it is unlikely that the department would approve an application for an additional facility".

At the time of this email, the Department of Enterprise was in partnership with the National Trust and Moyle Council on plans to replace their existing visitor facilities with a new publicly- owned centre.

This project was shelved last month by DUP Enterprise Minister Nigel Dodds, Mrs Foster announced she was "of a mind" to approve Mr Sweeney's rival plans.

Critics of Mrs Foster's provisional verdict have argued that it would lead to two visitors' centres at the Causeway - the premises currently operated by the National Trust and Moyle Council and Mr Sweeney's new commercial development.