Panelled wooden ceilings, tall lattice windows and a grand, sweeping staircase – these photos reveal for the first time the lavish interior of a “derelict” Victorian mansion that more than 5,000 people are trying to save from demolition.

Despite lying empty on the edge of Hampstead Heath for well over a decade, the understated grandeur of Athlone House, with its vast rooms and ornate stone pillars, remains fully intact.

Its historic original features are covered in a thick sheet of dust, but it is still easy to picture nurses moving up and down the corridors during the years it served as a hospital, and military spies working at desks while it was used as an RAF intelligence base.

Campaigners fighting to protect the Highgate mansion released these images this week to bolster their renewed attack on Camden Council, which is accused of failing to seriously resist plans for its demolition.

The Athlone House Working Group faces an upcoming legal battle as developers appeal against the council’s decision last month to refuse planning permission for a new eight-bedroom family mansion, described by objectors as an “Arabian nightmare”.

But the planners stopped short of blocking demolition of the historic building, saying the council’s hands were tied by a planning inspector’s report three years ago.

The inspector said that Athlone House could theoretically be replaced, but campaigners believe the council could have taken a stronger stand against this decision, alongside English Heritage.

‘Unforgivable’

Athlone House Working Group chairman Michael Hammerson – who took the pictures seven years ago but has only now made them public – said: “The photos show that there is tremendous detail and original structure left in the house and what needs to be restored can be restored, without too much trouble.

“When I took the photos, the house was in fairly good condition and now it’s covered in pigeon droppings. It’s unforgiveable and unacceptable.

“I think even we would concede that it needs radical upgrading, but there is a lot of scope for sensitive renovation.”

The council has argued that it is legally powerless to prevent demolition after a government planning inspector ruled in 2011 that the unlisted building made a limited contribution to the Heathside Conversation Area and that a high-quality replacement building could be acceptable.

The council also says it is unable to enforce a historic agreement requiring developers to restore the house to its former glory while a planning application is being considered.

However, the working group insists that the council has been “misadvised” on both of these issues, and has consequently “unjustifiably surrendered” the house’s demolition to developers.

Cllr Phil Jones, Camden’s cabinet member for planning, said: “The council has resisted demolition all along and the council has turned down the latest application. We’ve gone as far as we can go but, unfortunately, the planning inspector is above us in the hierarchy of decision-making.

“We have done everything we can to see renovation. I think that the developer has behaved disgracefully.”

A spokesman for developers Athlone House Ltd declined to comment.