The 23,000 acre Lake Vyrnwy estate is for sale with a price tag of £11 million.

The sale includes agriculture, forestry and commercial property around the lake – but not the lake itself.

Clive Hopkins, head of farm and estate sales at Knight Frank, which is handling the sale for Severn Trent Water, said: “Never before has a property of this size come to the open market in one block under a single ownership, not in my lifetime anyway.”

A Severn Trent spokesman said the company had been reviewing the ownership of the estate for a while.

“It is the largest land sale in the UK this year,” he said.

“It does not include the reservoir only the land around it. We will have the reservoir, dam and water supply.

“We have a few hurdles to go through before it is signed off.”

The tenants are said to be aware of the sale and Severn Trent officials will hold a consultation for 12 weeks.

Simon Baynes, Powys County Councillor, said he had heard of the sale announcement and hoped residents would be cared for.

He said: “From my point of view it is a threat and an opportunity.

“We have got to make sure whoever the buyer is looks after the local people.

“I grew up there so it is of particular interest to me.

“It is a major tourist attraction in Wales.

“My primary concern is the people who live there, through generations, are looked after.”