A British team of veteran ex-special forces soldiers summited Everest on Wednesday, expedition organisers said, in a bid to fly from London, climb the highest peak and return home within seven days.

Four men including government minister Alistair Carns left London on Friday pre-acclimatised to the low oxygen at high altitudes -- including using xenon gas, a method that raised eyebrows in the mountaineering community.

The men, who slept in special low-oxygen tents before departure from Britain, are raising funds for veterans' charities.

"All four of them, along with a photographer and five Sherpa team reached the summit this morning at 7:10 am," expedition organiser Lukas Furtenbach, of Austria-based Furtenbach Adventures, told AFP.

The team, which also includes Garth Miller, Anthony Stazicker and Kevin Godlington, are now descending from the 8,849-metre (29,000-foot) peak.

"They will descend to the base camp by evening and weather permitting will be back home within seven days," Furtenbach said.

The team is raising money for children whose parents were killed in conflict.

"I've seen, on multiple operations in Afghanistan, individuals who haven't returned," Carns, 45, who carried out five tours of Afghanistan, said before his departure.

Carns, a colonel in the Royal Marine Reserves, is the most highly decorated British lawmaker since World War II.

- 'Climb higher faster' -

The men are not the fastest to ascend Everest -- that record is held by Nepali climber Lhakpa Gelu Sherpa, who climbed from base camp to the summit in 10 hours and 56 minutes in 2003.

But expedition leader Miller, a commercial airline pilot, said it was a "new way of climbing 8,000-metre peaks".

Speaking before the ascent, he said they used "the latest sports science" to hone their physical preparation to allow them to "climb higher faster".

For decades the dream of reaching the summit of Mount Everest has required at least two months on the mountain doing a series of acclimatisation rotations.

But the team took a different route, heading directly to Everest's base camp on Saturday by helicopter, straight after arriving from London.

Areas above 8,000 metres are known as the "death zone" because thin air and low oxygen levels heighten the risk of altitude sickness.

They pre-acclimatised at home using hypoxic tents and special training techniques, before being administered xenon gas two weeks before departure.

The World Anti-Doping Agency banned the use of xenon in 2014, saying it could illegally enhance the performance of athletes.

- Criticism -

Furtenbach said xenon allows faster climbs and decreases the risk of altitude sickness.

"I was looking for new ways of acclimatising," Furtenbach told AFP.

Inhaling the gas prompts the production of the hormone erythropoietin (EPO) in the body, which encourages the formation of oxygen-carrying red blood cells to improve performance.

"You can say that xenon inhalation mimics the effects of a classical rotation to high altitude," said Michael Fries, a German doctor who works with Furtenbach.

In January the medical commission at the International Climbing and Mountaineering Federation -– the umbrella group for mountaineering organisations -– cautioned against using xenon.

"There is no evidence that breathing in xenon improves performance in the mountains and inappropriate use can be dangerous," it said.

It also comes at a financial cost, with the climb costing an estimated $170,000 per person, according to Furtenbach, several times higher than the slower expeditions.

Some have been critical of the use of xenon.

"Mostly I view it as a stunt," said Adrian Ballinger, who runs US-based Alpenglow Expeditions, a company that has worked to develop accelerated acclimatisation methods.

"To me, those things take away from what makes climbing Mount Everest unique, which is the unknown outcome and the fact that each person on the mountain is pushed to their emotional, physical and mental limits."

Furtenbach, who has tested the gas on mountain climbs since 2020, said he hoped that the expedition would help normalise xenon to become "part of a standard safety protocol for high-altitude mountaineering".

pm/pjm/pst