The mysterious owners of a historic Highgate mansion have announced they will take their six-year battle to demolish the former hospital to the High Court.

The Planning Inspectorate last month dismissed an appeal to knock down and rebuild Athlone House as a luxurious eight-bedroom palace on the edge of Hampstead Heath in what was hailed as a landmark victory for 5,000 campaigners who fought to save the crumbling Victorian pile.

But rather than give up, developers Athlone House Limited have come back fighting, and last Wednesday pledged to take the decision to judicial review.

A spokesman for Athlone House Ltd said: “We believe that there are strong grounds on which to challenge the inspector’s decision and that is why we are applying for a judicial review of the decision to refuse planning permission.”

The revelation has angered conservationists of the Athlone House Working Group, who want to see the former RAF intelligence base restored to its former glory.

Highgate Society vice-president and lead campaigner Michael Hammerson said: “To say it is disappointing does not begin to express our views.

“We are appalled at what we can only describe as an attempt to exploit the planning system by people who have the money to do so.”

He added: “We would have hoped they would do the honourable thing and restore the house.”

Athlone House Ltd is obligated to restore the house under a historic planning agreement made when planning permission was granted for luxury flats in the grounds of the mansion.

But Camden Council has been unable to enforce the restoration because of a legal loophole.

The developers have now tried twice to knock down the house, having lodged two planning applications since 2009.