Two TA soldiers who died while attempting to get into the special forces were on the first day of a week-long assessment when they collapsed with suspected heat exhaustion, according to Sky sources.
Police are investigating the deaths of the two servicemen on the hottest day of the year while a third serviceman is in a serious condition in hospital.
The three were part of a group training in the Brecon Beacons, some of Wales' most rugged terrain, on Saturday when temperatures topped 29.5C (85.1F).
It is understood that the men were on the first day of a week-long assessment as part of the Territorial Army section of the SAS.
The selection process was running alongside regular SAS trials and has now been abandoned due to the deaths.
For the regular version of the special forces elite, the trials would involve a 40-mile speed march with about 55lb on their backs, including a replica rifle, normally completed over a 20-hour period.
It is likely that the process would be slightly reduced for the TA version or split up, but nevertheless gruelling.
Sky News Defence Correspondent Alistair Bunkall said the "very rigorous selection process" would have meant "long yomping walks carrying an awful lot of weight".
Police are investigating the deaths
With military sources indicating soaring temperatures may have been to blame for their deaths, Bunkall added that the investigation into their deaths was like to focus on the climate and whether enough preparations were put in place.
Colonel Richard Kemp, former commander of British troops in Afghanistan, pointed out the difference between the selection processes for the regular Army and the special forces, where the onus is on individual achievement and self-motivation.
He told Sky News: "The Army relies a lot on the buddy-buddy system, in other words of either a commander or fellow soldier looking out for their mates to see if they are exhibiting any symptoms of heat illness.
"Of course if they are doing SAS selection ... then a lot of that is done as individuals."
Chris Hunter, a former special forces bomb disposal officer who was training for a marathon on the Brecon Beacons over the weekend, told Sky News that every member of the armed forces is taught how to survive in extreme heat as soon as they join up - using techniques such as carrying lots of water, rehydrating with salts and adding and removing layers of clothing as necessary.
But he added: "Of course when they are trying to meet very tight timelines, carrying lots of equipment, literally speed marching up the mountains, running down them and navigating at the same time it can be quite difficult to do so."
Mr Hunter said a medical assessment by a doctor is always carried out before "any extreme training" and "the individual soldiers would certainly have been physically fit".
The Ministry of Defence and police are both investigating what happened. The soldiers' families have been informed.
The MoD has refused to confirm that the three were members of the TA aspiring to join the reservists' branch of the SAS.
And a spokesman said there are no plans to change "routine exercises" in light of the incident.
The Brecon Beacons is one of several locations British military use as part of their training. The deaths occurred near the Storey Arms activity centre.
The area's rugged and sprawling terrain helps prepare soldiers physically and mentally for warfare as well as put their logistic skills to the test, making it an ideal area for elite forces personnel like the SAS.
However the Beacons' jagged topography can prove dangerous even to the most hardened and physically fit.
Earlier this year, an army captain was found dead on a snow-covered Corn Du mountain.
It was thought that Rob Carnegie had been taking part in a gruelling 17-40 mile march in freezing conditions in the Brecon Beacons as part of a selection process for the special forces regiment, when he collapsed and died.
However, this time investigators are examining whether hot temperatures played a major factor.
The soldiers who died were thought to be trying out for the SAS
A source said: "It is a case of the people succumbing to being affected by the training that they were doing."
The Army's website said the Brecon Beacons were used because they are so demanding and prepare soldiers for the "extraordinary things" they have to do on deployment.
The website says: "Training for high-intensity, light-role war fighting is the way soldiers and officers are prepared for any operational situation they may face - conventional war, counter insurgency, security sector reform, peacekeeping or supporting civil authorities.
"This ensures that the training is as close to current operations and pre-deployment training as possible, whilst maintaining the ability to train for high intensity war fighting."
News of the deaths has been met with shock in the nearby town of Brecon, which is home to The Infantry Battle School.
Brecon mayor and Powys county councillor Matthew Dorrance said: "It's incredibly sad for the friends and family of the people who have lost their lives and our thoughts are with the person who is injured.
"In one way we've been blessed with the weather but for people working in this heat, they're tough conditions."
Members of all four of South Wales' mountain rescue teams said they had been called out to assist when the two servicemen died.
Thirty members of Central Beacons, Brecon, Western Beacons and Abergavenny-based Longtown Mountain Rescue Teams joined the operation near Pen y Fan, which is the highest mountain in south Wales.
Mark Moran, from Central Beacons MRT, paid tribute to its members who took part in the rescue operation.
"They are all volunteers, who are highly trained and dedicated" he said.
"We were working alongside military personnel who remained extremely calm and professional during this tragic incident. Our thoughts are now with the families of those involved."