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James Hopkinson, a former instructor at the Infantry Battle School in Brecon, has described the challenging conditions that the two soldiers who died would have been up against.
He said that military exercises in the Brecon Beacons were designed to push soldiers "to the edge of their endurance," but that measures would "undoubtedly" have been put in place to mitigate the effects of the hot weather.
He added that while the exact circumstances of the deaths remain unclear, such endurance training is needed to make the British Army "the best in the world".
This photo shows Lance Corporal Craig Roberts, who died during a training exercise in the Brecon Beacons on Saturday.
Photograph removed due to copyright agreement.
The two soldiers who died on the Brecon Beacons on Saturday were taking part in a selection process for the SAS reserves, sources have told the Press Association.
According to the SAS website, the elite force has two reserve units which recruit male personnel from both military and civilian backgrounds.
The website states that they are "tasked to the highest level" and must complete a two-phase selection process demanding about 80 days over 12 months.
The initial 'Aptitude' test is described as "a progressively arduous phase, requiring greater expenditure of physical energy, designed to select volunteers with the right qualities".
Another walker who saw some of the soldiers training in the Brecon Beacons on Saturday has said he saw nothing out of the ordinary. Phil Speck told BBC News:
The Ministry of Defence has named one of the soldiers who died on a training exercise in the Brecon Beacons in Wales as Lance Corporal Craig Roberts.
The other was identified only as "another Army reserve soldier".
The soldiers' families have been informed.
Major Alan Davies told the BBC that the Brecon Beacons is one the most challenging terrains military personnel can encounter:
"On one end of the spectrum you have cadets being taken for mountain walking and at the other end of the spectrum the SAS use it," he said.
He added that the three men may have been carrying very heavy equipment and working to a deadline, which meant they would have been pushing themselves very hard.
The deaths are thought to be related to the hot weather but despite more high temperatures expected an MoD spokesperson told the paper: "There no plans to change any planned exercises, whatever their nature, in light of this."
The two soldiers who died during a training exercise in the Brecon Beacons were serving with the Territorial Army, according to the BBC.
Defence sources suggested the personnel had "succumbed" to unusually hot weather conditions during the drills in Wales.
The TA is a British volunteer reserve force which recently announced plans to increase its manpower to 30,000.
An investigation is underway into the death of two servicemen on a training exercise in the Brecon Beacons on Saturday.
No live ammunition was involved and a military source suggested the personnel had "succumbed" to the conditions on the hottest day of the year.
This video report is by ITV News' Ben Chapman:
Latest ITV News reports
Emotional tributes have been paid to a third army reserve soldier who died from injuries sustained during SAS selection training.
One of two Army reservists killed in a military exercise in the Brecon Beacons on Saturday has been named as Lance Corporal Craig Roberts.