Two military personnel have died during a training exercise, the Ministry of Defence said, with a third in a serious condition.
The servicemen were on a routine exercise on Saturday on the Brecon Beacons.
The incident is being investigated by Dyfed Powys Police as well as the MoD.
A spokesman said: “The MoD can confirm that it is working with Dyfed Powys Police to investigate an incident during a training exercise on the Brecon Beacons on Saturday in which two members of military personnel died.
“The two servicemen’s next of kin have been informed. More information will be released in due course.”
Police confirmed a third man was involved and was in a serious condition in hospital.
Saturday was the hottest day of the year for many parts of the country, and temperatures in Usk in nearby Monmouthshire, South Wales, reached 29.2C (84.5F).
A source said: “It is a case of the people succumbing to being affected by the training that they were doing.”
The military use the Brecon Beacons for a wide range of exercises for various Army personnel.
The Infantry Battle School (IBS) is based at Brecon and the tough, demanding landscape is used to put regular and special forces soldiers through their paces.
The Army website says the Brecon Beacons are 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.
“Training at IBS is delivered here by high-quality instructors with a wealth of operational experience, and rated in the top third of the Infantry.
“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. IBS delivers competent and confident commanders for the Field Army by running command and leadership training, infantry tactics training, weapons training, and live firing range qualifications. It also provides specialist training teams to assist foreign forces in their development, and allocates some places on courses for overseas students.”
The Brecon Beacons is one of several locations British military use as part of their training. Its rugged and sprawling terrain helps prepare soldiers physically and mentally for warfare as well as putting 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 as part of a selection process for the special forces regiment, when he collapsed and died.
A spokesman for Dyfed Powys Police said: “A Ministry of Defence exercise was taking place in the area, and they are fully co-operating with our inquiries.
“The next of kin are being informed, and HM coroner has been informed.”
David Davies, the Conservative MP for Monmouth, said: “It’s a terrible tragedy.”
A former member of the Territorial Army, he added: “Nobody should jump to any conclusions here about what may have caused this.
“The training is there for a purpose, and it is arduous training at any level, and sometimes things tragically go wrong.”