A soldier who died while training in Brecon, south Wales, had just finished an eight-mile fitness test, the Ministry of Defence has said.
Cpl Joshua Hoole, 26, of Ecclefechan, near Lockerbie, died during pre-course training for the Platoon Sergeants' Battle Course.
Ambulances were called to Dering Lines infantry training centre in Brecon shortly before 09:00 BST on Tuesday.
The MoD said he collapsed after arriving back at the barracks.
Cpl Hoole was serving in the Rifles regiment at ITC Catterick. The cause of his death is not yet known and police are investigating.
An MoD spokesman said: "It was a two-week course and they were on day two.
"They all did the annual fitness test which is an eight-mile route carrying 25kg in two hours. No-one can do the route in less than one hour [and] 50 minutes.
"The annual fitness test is the first thing you do. This test gets done all across the infantry and is not supposed to be taxing."
The spokesman said the officer in charge would have checked the weather conditions and the temperature as part of his duties, on what was the hottest day of the year so far in Wales.
"It was 17 degrees when they started and 21 or 22 degrees when they finished," the spokesman said.
He said the Army had looked at how training was carried out following previous deaths - including the three soldiers who died during a SAS selection exercise in 2013.
"Where practicable, every recommendation and report has been implemented in order to ensure soldiers training is made as safe as possible," he added.
'Heart of gold'
Tributes have been paid to Cpl Hoole, who was due to get married and be the best man at his brother's wedding.
His grandfather, John Craig described him as a "beautiful grandson" and a "super fit boy."
He said: "He was a dedicated soldier," adding he "always wanted to be top dog".
His aunt, Morag Craig, told BBC Wales that the family will demand to know why he died.
"We are going to ask that question," she said.
She described Cpl Hoole as "absolutely fantastic" and "always there for any of us in the family".
"There was nothing he wouldn't do for you - he had a heart of gold."
"I think we are hoping that we'll wake up and someone has been playing a joke," she said, "but now it's starting to hit home."
His step mother Carol Jeffreys posted on her Facebook page: "Rest easy soldier. My beautiful stepson. Can't believe it. Will always love you.
"So very proud of you. Thoughts with your family and fiancee."
Friend Kyle Blackburn said: "We are all going to miss you, there was never a dull moment when you were around.
"Always had something to say to me even though it was cheek. Sleep tight, always be proud to call you my mate."
Dyfed-Powys Police said it was working closely with the MoD and Cpl Hoole's next of kin had been informed.
An ambulance service spokesman said it was called to the infantry training centre at Dering Lines "to reports of a medical emergency". The Wales Air Ambulance also attended.
The Health and Safety Executive said it had been informed of the death.
A spokesman added: "Dyfed-Powys Police are leading the initial enquiries into the soldier's death and HSE will not speculate at this stage on the exact circumstances of this fatal incident."
Defence Minister Harriett Baldwin told the House of Commons defence committee she wanted to express her sorrow at the "very sad death" and to "make the commitment that the Ministry of Defence will, of course, be undertaking a full investigation".
Cpl Hoole's death comes three years after three soldiers died during an SAS training exercise in the Brecon Beacons on 13 July 2013, one of the hottest days of the year.
L/Cpls Edward Maher and Craig Roberts died on the beacons after suffering heatstroke during a 16-mile (26km) SAS test march on 13 July 2013.
Cpl James Dunsby died from multiple organ failure in Birmingham's Queen Elizabeth Hospital more than two weeks later.
A coroner ruled neglect played a part in their deaths.
L/Cpl Dunsby's widow, Bryher Dunsby, offered her condolences to Cpl Hoole's family.
She said: "I would like to offer my heartfelt condolences and my love to Joshua's fiancée, family and friends following another tragic death in training at Brecon.
"I sincerely hope that all the inquiries into this incident are conducted promptly, with truth, integrity and total transparency by the MoD."
The Platoon Sergeants' Battle Course is a promotion qualifying course to the rank of Sergeant for Rifle Company Platoon Sergeants, the MoD website says.
It is said to be both mentally and physically demanding but students "receive the highest level of tuition and testing to ensure they are fully prepared for the rigours of command".