The Welsh-born Oscar winner made his donation a week after the National Trust launched its Save Snowdon Campaign, the public appeal to help the trust buy the part of the Welsh mountain that has been put up for sale for £3m.

Sir Anthony, president of the trust's Snowdonia Appeal since its launch in 1990, said: "Snowdonia is one of the most beautiful places in the world and Snowdon is the jewel that lies at its heart. It must be cherished and protected.

Sir Anthony will visit Snowdon at the end of August

"The National Trust is dedicated to the preservation of this land and its natural heritage. I know this and have seen it first hand. I am very pleased that I am able to offer this support."

He said he would visit the mountain at the end of August.

Richard Cuthbertson, chairman of the trust's Committee for Wales, said it was "totally overwhelmed by Sir Anthony's generosity".

'Magnificent gesture'

"It is an enormous boost to the appeal. The National Trust has been profoundly grateful for all the help which Sir Anthony has given the Snowdonia Appeal since its launch in 1990.

"This latest magnificent gesture demonstrates true leadership and provides a firm foundation for our campaign fund to save Snowdon."

The trust was given just 100 days to raise the money to buy the 4,000-acre estate on the southern flank of the 3,560ft peak. Phone lines have been set up to take donations, and Chris Brasher - the conservationist and founder of the London marathon - set the ball rolling by giving £200,000.

Race against time

"We do need to keep the money rolling in," he said on the day Sir Anthony pledged his million. "We only have 93 days to go before we need to complete the deal."

The National Trust is now expected to submit a bid to the National Lottery Heritage Fund for cash to help secure cash needed to buy the section of the mountain that is on the market.