The National Trust has launched an urgent public appeal to raise £2.6 million to help it buy a "magical" stretch of coastline for the nation.
The trust wants to buy Bantham beach and the Avon estuary in south Devon, to maintain quality access to the sandy beach for the hundreds of thousands of people who visit each year and to protect the landscape along the unspoilt coastal site for nature.
The beach, nestling in the South Hams, has panoramic views over Bigbury Bay and Burgh Island which, with its Art Deco hotel and access by "sea tractor" at high tide, is the famous setting for Agatha Christie novels and TV adaptations.
If successful, it would be the National Trust's most expensive coastal purchase.
But a failure to secure the beach - described as the best surfing beach in south Devon - and the estuary for the nation could leave its future uncertain, it warned.
The trust fears access to the beach could be reduced and the coastline could be at risk of inappropriate development if another buyer snaps up the estate, which is on the open market.
The Trust has already been able to commit £4.6 million from its Neptune campaign to protect coastal areas, but urgently needs to raise another £2.6 million to buy the beach and estuary and pay initial management and conservation costs.
If it buys the land, it plans to maintain access for visitors and conserve the landscape of the flooded river valley, enhancing the oak woodlands that sweep down to the estuary and sowing wildflower rich meadows.
Mark Harold, National Trust director for the South West, said: "This is a magical place, a true jewel on the South West coast.
"We now have a once in a lifetime opportunity to permanently secure and protect the landscape of one of the least spoilt coastal sites and secure its future for everyone to enjoy.
"If we don't raise the money then the future of Bantham beach and the Avon estuary is uncertain and this stretch of coastline might one day be disrupted by inappropriate management or development."
Until now, Bantham's owners have protected the unspoilt nature of the estuary and surrounding countryside, which is in an Area of Outstanding Natural Beauty, and avoided inappropriate commercialisation of the beauty spot.
The entire estate, which has been in the same family for generations, is on the market for £11.5 million but the National Trust has decided not to bid for the cottages in the village of Bantham, saying its core purpose was not to be a social landlord.
If the estate were sold to a private owner, some level of access to the stretch of coastline would remain through the South West Coast Path.
But access to the beach via a private road could be significantly reduced if a purchaser sought exclusive access and closure of the car park, the trust said.
Joel Wakeling, a National Trust ranger who is from south Devon, said: "When I think of Bantham I think of the golden sand and rolling Atlantic waves.
"Somewhere that I spent a lot of time when I was growing up, it's a place that means so much to so many.
"As someone whose heart and soul is connected to this very special stretch of coastline it would be amazing if the National Trust could raise the money to buy Bantham beach and Avon estuary.
"I would then know that my children and children's children would be able to have the same experiences that I had growing up, and that everyone will be able to enjoy this special place forever."
The proposed purchase would add to the 742 miles of coastline around England, Wales and Northern Ireland, including beaches, cliffs, islands, sand dunes and estuaries, which is cared for by the National Trust.
:: People can donate to the appeal online at www.nationaltrust.org.uk/lovebantham, by calling 0344 800 1895, by texting BANT14 followed by the amount they want to give to 70070, or by cheque made payable to "National Trust" - with "Bantham appeal" written on the back - and posted to National Trust, Supporter Services Centre, PO Box 574, Rotherham S63 3FH