Leaders of the National Parks which make up the lion’s share of the Prime Minister’s constituency have issued an extraordinary appeal to Rishi Sunak, saying his government’s plan to relax planning rules in protected areas would cause “irreparable and substantial harm”.

In a joint letter to Mr Sunak, Yorkshire Dales and North York Moors National Park chairmen Neil Heseltine and Jim Bailey said allowing hundreds of buildings such as barns, cafes and offices to be converted into housing without planning consent could cause “a wide range of harmful and permanent impacts” to his constituency.

There are an estimated 6,200 traditional farm buildings in the Dales alone, and the stone field barns have been a particularly protected feature of areas like Swaledale and Wensleydale.

The unprecedented appeal follows Government officials underlining there would need to be “watertight reasons” given in a consultation over its Levelling Up Bill proposals to abandon the legislative move which is aimed at speeding up housebuilding.

It also follows Mr Sunak drawing criticism from climate change campaigners for announcing a series of U-turns on national climate change targets and hailing the approval of an environmentally damaging fossil fuel project in the North Sea.

The Government said granting permitted development rights on an array of properties in protected areas would give certainty for people wanting to develop homes and remove time and financial barriers in submitting a planning application.

While some Conservatives in the county have privately voiced partial support for the proposals, many other people believe they would be disastrous for the protected landscapes.

Mr Heseltine and Mr Bailey told the Prime Minister they had “grave concerns” as the changes could see sensitive landscapes littered with many more second homes and holiday lets.

The letter says: “These changes, in our view, will cause huge damage to nationally protected landscapes and their economies whilst being counterproductive to many of the Government’s own policy goals for healthy rural communities.

“People will rightly ask why, in an area with national protection for its natural beauty, planning permission is needed for minor developments such as a domestic side extension or a porch over a certain size and yet an agricultural business is able to develop up to ten houses in the open countryside without needing to apply for planning permission.”

The two chairmen said the move would be “deeply unpopular” and bring the planning system into disrepute with a further loss of public confidence.

Their letter adds: “The impact on walking in the valleys of Upper Swaledale and across the Cleveland Hills to the Esk Valley, the route of the iconic Coast to Coast Walk should these proposals be enacted, doesn’t bear thinking about.

“New conversions will bring new power lines, roads, domestic gardens, additional lighting, play equipment and other residential paraphernalia into otherwise traditional pastoral landscapes, which Parliament has committed to protect for the nation as places of great natural beauty.”

The chairmen wrote that, in the absence of planning rules, the market would dictate a building’s use, with most, if not all, of the conversions ending up as second or holiday homes, meaning they would be beyond the reach of most local people.

The letter states relaxing planning rules would be likely to lead to an accelerated loss of local services in villages, with buildings converted into yet more second or holiday homes.

A spokesman for Mr Sunak declined to comment when asked about the claim that the changes would be profoundly damaging to his own constituency.

A Department for Levelling Up, Housing and Communities spokesperson said: “Our public consultation has closed and we will now consider all responses. We have been clear that any developments must be considered sympathetic to the surroundings, enhance the environment and meet the needs of people in the local community.”

Sunak warned of ‘huge damage’ on horizon in own constituency