Energy company Axpo, which runs the plant in northern Switzerland that first began operating in 1969, said in a statement that it had decided "to secure the operation of the Beznau nuclear power plant until 2033".

When it is finally taken offline, the installation will be 64 years old, the company said in a statement.

Switzerland has placed no maximum life span on its nuclear power plants.

The four currently in service, which provide around a third of Switzerland's total electricity production, can run for as long as they are considered safe.

Axpo said it had conducted "a comprehensive review" before determining the cut-off time for Beznau, and that "safety was the top priority in all considerations".

It said it would keep one of the two units on the grid until 2032, with the other unit staying on until 2033, adding that it planned to invest an additional 350 million Swiss francs ($396 million) in the plant.

Since the start, it said it had invested more than 2.5 billion francs in upgrading and modernising the two units.

The Swiss Energy Foundation (SES) said setting the cut-off date was "logical", after Swiss voters in June approved a law accelerating the development of renewable energies as part of efforts to attain carbon neutrality by 2050.

The country's Green Party described the scheduled shutdown as "an important step towards Switzerland's definitive exit from nuclear", according to the domestic Keystone-ATS news agency.

The Swiss approved the gradual phase-out of nuclear power in a referendum in 2017, by banning the construction of new power plants.

That law was the result of a long process initiated after the 2011 Fukushima nuclear accident in Japan, triggered by a tsunami.

Greenpeace, which has been calling for Axpo to swiftly shut down the two reactors at Beznau, slammed Thursday's announcement.

It warned that keeping the ageing installation alive was dangerous, and insisted that swelling renewable energy production had rendered Beznau's output "redundant".

"Maintaining the activity of the oldest installation in the world is a risky and useless experiment," Georg Klingler of Greenpeace Switzerland said in a statement.

"Beznau must be shut down immediately."

nl/rjm/bc

SES SA