ALLOWING development to go ahead near York's ancient Askham Bog would be a desecration on a par with building next to York Minster, naturalist Sir David Attenborough said.

During a walkabout at the ancient peat bog as part of a visit to mark the Yorkshire Wildlife Trust's 70th anniversary, Sir David was told there had been a proposal, in a previous draft of the York local plan, to release land on the site's northern edge for a large housing development.

The great naturalist drew himself up and let loose.

"If someone was proposing to put a building site next to York Minster there would be an outcry," he said.

"For naturalists, that's what this site is like. This is a treasure that is irreplaceable. The citizens of York are lucky to have it on their doorstep and it is their responsibility to look after it."

The bog, formed thousands of years ago in a scooped-out depression left behind by retreating glaciers at the end of the last ice age, was the Yorkshire Wildlife Trust's first nature reserve.

It was presented as a gift to the Trust's precursor, the Yorkshire Naturalists Trust, by Sir Francis Terry and Arnold Rowntree 70 years ago.

It remains one of the jewels in the Trust's crown - a diverse nature reserve that contains more than 300 plant species, including the rare royal fern.

During his walkabout, Sir David came across one of the great ferns. It was probably several hundred years old, he was told.

He examined it closely. "I must say, that's quite impressive," he said.

The 90-year-old naturalist also met a group of wildlife trust staff and volunteers doing a species survey.

"What are you going to show me?" he asked.

The trust's landscape officer Jo Milborrow explained they were doing a count of 20 'indicator species' to see what effect using Exmoor ponies to graze the bog was having on plant diversity.

Sir David's face lit up.

"I can remember doing the same myself at Wicken Fen (in Cambridgeshire) when I was about 14!" he said. "I got the shape of all the plants imprinted on my hands!"

After the walkabout, Sir David was due to attend a Yorkshire Wildlife Trust 70th birthday reception at the University of York.

But he had one last word on the importance of sites like Askham Bog.

People of all ages and all backgrounds should visit it, he said, and learn to understand and appreciate what makes it precious.

"The natural world is coming under huge pressure," he said. "If we're going to deal with it, it will cost money. People need to understand that it is essential to humanity."