A clean-up of the vandalised Cerne Giant is going to be more difficult and costly than first thought, the National Trust admitted today.

The night-time paint attack left the chalk figure's world famous genitalia purple.

Trust spokesman Alex Brennan said: "The paint is not a water-based one and is going to be difficult to remove. In the short-term it's been covered with a limewash to restore the colour but that won't last. Rain will wash it away before long.

"We are dismayed by this vandalism. There have been publicity stunts at the giant before but nothing like this where it's left an indelible mark. It's going to take time and manpower to remove it."

Robert Rhodes, the trust's head warden for West Dorset, said: "This is a terrible case of vandalism to the giant.

"The giant is a scheduled ancient monument of national importance and this wanton vandalism is the equivalent of spraying graffiti on Stonehenge or St Paul's Cathedral.

"As a charity we will now have to expend our valuable time and resources attempting to restore the giant to its former appearance.

"The grassland surrounding the monument is a Site of Special Scientific Interest so the use of chemicals to remove the paint could potentially damage the habitat."

He was also concerned about causing erosion by removing the paint.

Mr Brennan said the trust now had to liaise with various statutory organisations to decide how to restore the giant.

A campaigner from the Fathers 4 Justice group is believed to be responsible for disfiguring the famous landmark.

The vandal described himself as the Westcountry's Purple Phantom. Written beside the giant were the words Read Family Court Hell, a reference to a book recently published by Fathers 4 Justice campaigner Mark Harris.

Mr Brennan said: "The police are making inquiries and I hope they catch the person responsible. At this stage our priority is to get the monument back to the condition that people want to see it in."

In July the cartoon character Homer Simpson was painted on to grass in a field next to the 180-foot tall giant to promote the new Simpsons film.