The government could face a "protracted and complicated planning battle" if it decides to push ahead with a mega-prisons plan at Wethersfield.

Campaigners who want to stop the Ministry of Justice's plan to build two prisons for nearly 3,500 inmates in Essex have written to justice secretary Dominic Raab.

They have told Mr Raab they believe that the MoJ's public consultation principles were breached, that the proposal goes against the MoJ's own guidelines for locations of prisons, and that the proposal is contrary to national and local planning policy affecting habitats and heritage.

If built, the category B and C men's prisons could feature four-storey blocks on a former Cold War base.

The MoJ claims that the units would help the country meet a target of 18,000 new prison places by the mid-2020s.

Rosie Pearson, a Stop Wethersfield Airfield Prisons campaigner, has told Mr Raab that the site is too "isolated".

She said: "The site is far from jobs, far from homes, and a long way from urban areas where crimes are committed."

Rosie wrote: "This site should not be under consideration for prisons.

"Unfortunately, a protracted and complicated planning battle will ensue if prison proposals are not withdrawn."

Rosie said that Wethersfield Airfield is rich in wildlife and important as a site of historical importance.

She added that it could prove costly to move staff and prisoners between Wethersfield and other prisons sites.

An MoJ spokesperson said: "We want to work closely with Braintree residents before submitting a planning application, which is why we consulted widely and held online and in-person information sessions.

"We are now considering responses and Braintree District Council will hold its own consultation, if a planning application is submitted."

Braintree MP James Cleverly has previously raised his concerns at ministerial level with the MoJ.

Councillor Graham Butland, Braintree District Council leader, said: "BDC has not yet received a formal planning application for the proposals.

"Given its potential role in any future application, it will not have a view on whether it 'supports' the proposed prison or not until that planning process has concluded."