An infantry soldier from Ecclefechan, in Dumfries and Galloway, has died while on a training exercise on the hottest day of the year.
Josh Hoole, of The Rifles regiment, died on Tuesday in the Brecon area of Wales, where temperatures peaked at just above 30C.
His grandfather, John Craig, told ITV Border that Josh had toured Afghanistan twice, and was due to get married next year.
He was supposed to have been best man at his brother's wedding next Saturday.
Josh Hoole was based at the Infantry Training Centre in Catterick, North Yorkshire, and was on pre-course training for the Platoon Sergeants’ Battle Course.
The Ministry of Defence says he was taking part in a basic combat fitness test, which everyone who is on the course has to complete.
It involves an eight mile march wearing boots, and carrying webbing, a burgen and rifle. The total weight is 25 kilograms.
The march started at 7am at Dering Lines, and followed tarmac roads around Brecon. The Ministry of Defence says there were no hills on the route.
The march was two hours long, and when returning to Dering Lines Josh Hoole collapsed and an ambulance was called.
His death follows those of three soldiers who were taking part in an SAS training exercise in the Brecon Beacons on one of the hottest days of 2013.
The Ministry of Defence will carry out a full investigation into Josh Hoole's death, Defence Minister Harriett Baldwin said.
She told the House of Commons Defence Committee that 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 and will share that with the committee".
The Platoon Sergeants' Battle Course is taken by infantry soldiers who want to progress to the rank of sergeant.
The course is run three times a year, with the next one set to take place in August.