The Giant's Causeway, one of the UK's most spectacular geological features, is at the centre of a dispute over plans that could see the site reduced to what the National Trust describes as "little more than a theme park".

The local authority, Moyle District Council, is inviting bids for a "unique leisure opportunity" to transform nine acres around the existing visitor centre at the World Heritage Site in Co Antrim, Northern Ireland. Meanwhile, the province's environment department is backing a proposal to convert a listed 19th-century cottage overlooking the dramatic basalt columns into a pub, restaurant and car-park complex.

The National Trust says local officials have been lured by the prospect of multi-million-pound returns into breaking a pledge to keep development near the unique prehistoric landmark to a minimum. And the trust, which has owned the two-and-a-half mile (4km) causeway since 1961, says it will challenge any decision favouring the plans in the High Court.

Ruth Laird, the trust's director for Northern Ireland, said: "Commercial development has been given precedence over a unique natural environment. It is our duty to assess all avenues open to us to see this development stopped."

Mrs Laird said that any siting of a restaurant and/or "leisure" facility so close to the causeway would flout two recent written undertakings by the environment department's planning service. One document, Planning, Archaeology and the Built Heritage, said that, because of its World Heritage status, "development proposals for within a 4km radius of the site will be subject to particular scrutiny". The other, a letter from a local planning officer to a National Trust property manager, emphasised that no construction would be allowed within the 4km zone "unless it is within the wider public interest".

Mrs Laird said: "I am amazed that the planning service looks set to recommend approval for this scheme."

A trust source, referring to millionaire Peter de Savary's controversial transformation of Land's End into a theme-park-led attraction, said: "There's a fear that it could become the new Land's End. In 20 years' time will people be taking their children there to go to a theme park and have a burger? Oh, and have quick look at some basalt cliffs while they're at it."

A spokesman for Moyle council said: "The visitor centre burnt down last year and a temporary building is in its place. Everyone agrees we need a world-class centre. The council is saying it can't afford that without private help."