Britain's oldest national conservation body has joined mounting opposition to plans for a zip wire in the Lake District.

The Open Spaces Society says plans to stretch eight cables across the lake, with take-off and landing points and gantries, would "make Thirlmere into Zipper-mere".

The scheme, planned by Treetop Trek and dubbed Thirlmere Activity Hub, would feature two zip lines and an 18km-long family-friendly cycle way and would be located within the managed forestry around Thirlmere, with the lines stretching from one side of the lake to the other.

Kate Ashbrook, general secretary of the society, said: "The development would dominate this splendid landscape and destroy its peace."

The society argues the work would be a major development and go against the Lake District National Park Authority's statutory purpose to conserve and enhance the natural beauty, wildlife and cultural heritage of the national park.

Says Kate: ‘It is evident that there is no overriding national need for this, and certainly there are alternative sites outside the national park. This is an unacceptable commercial exploitation of a wild, sensitive area. Its international importance has been recognised in the recent World Heritage Status award.

Ms Ashbrook added: "The Open Spaces Society has a long history with Thirlmere. In 1878 a reservoir scheme was proposed by the Manchester Corporation by a private bill in parliament. By threatening opposition, the society persuaded the corporation to amend the bill and give the public a right of access on foot to the commons which were part of the scheme.

"This right was enshrined in the Manchester Corporation Act 1879 and endures today, further reinforcing the value of this area for quiet recreation. Indeed, Thirlmere is rightly considered to be the birthplace of the conservation movement, a special place indeed."

The society has submitted a letter of objection to the national park authority, which is considering the plans.

It has questioned whether the authority can legally grant consent for a development which is in breach of its statutory purposes and many of its policies.

Objections have previously been submitted by groups including Friends of the Lake District, the Campaign for National Parks and the British Mountaineering Council.

Television star Caroline Quentin this week joined those opposing the plan.

The scheme has also also received backing, with supporters including Cumbria Tourism and the Lake District Park Partnership Business Task Force.

Responding to the opposition, Mike Turner, managing director of Treetop Trek, said: "We have demonstrated unequivocally that our project will have no significant visual or noise impact in the Thirlmere Valley.

"Zip lines and cycle tracks are clearly not environmental matters - they are emotional ones - as clearly highlighted by the opponents decision to roll out a celerity to add weight to their argument.

"This debate is rapidly and sadly becoming about who should be using the landscape and what they should be doing in it, I for one want to see more people enjoying it."

The closing date for comments on the plan has recently been extended to January 12.