The Falmer stadium would accommodate 23,000 fans

The warning was made on Friday by Tom Oliver, from the Campaign to Protect Rural England (CPRE).

In a speech, he attacked a relief road in Dorset, a science centre in Kent and a football stadium in East Sussex.

However, the representatives behind the new road and buildings have defended their benefit to the community.

Speaking at the conference for The National Association for Areas of Outstanding Natural Beauty, Mr Oliver pinpointed the "threat" posed by the planned science centre in Wye, Kent, and the new relief road in Dorset.

He also condemned a planned football stadium in Falmer in East Sussex.

'Grossly irresponsible'

In his closing speech Mr Oliver said: "Many Areas of Outstanding Natural Beauty are under greater threat than ever before."

The proposals in Dorset, Kent and East Sussex are "the top three [but] there are lots of other threats", Mr Oliver told the BBC.

"They include possible expansion of flight paths over our most tranquil protected landscapes," he said.

The CPRE attacked Imperial College London's plans to build a �1bn science research hub at its campus in Wye, in the Kent Downs, accusing it of being a "grossly irresponsible proposal".

A science centre in Wye could create 12,500 jobs

College rector Sir Richard Sykes has said the development would be a "fantastic opportunity to explore some of the most exciting problems in science, engineering and medicine today".

The Weymouth Relief Road proposals in the Dorset AONB are also targeted for criticism.

However, the county council has insisted the 3.75-mile (6km) single carriageway route would "make a big difference to everyone who lives or travels in the Weymouth-Dorchester area".

The CPRE has also criticised plans for a new Brighton and Hove Albion FC stadium in Falmer as "an ongoing threat... on nationally protected landscape [the Sussex Downs AONB]".

The club's move was backed by Brighton and Hove City Council, and then approved by Deputy Prime Minister John Prescott in October last year.