Two soldiers have died during a training exercise on the Brecon Beacons in Wales, the Ministry of Defence has said.
The pair were among three army personnel thought to have been affected by heat exhaustion on Saturday, the hottest day of the year for many parts of the country.
A statement released by the MoD 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 but it would be inappropriate to comment further at this stage."
Temperatures in the Brecons are normally cooled by the 600m (2,000ft) and higher elevations that military personnel train at but on Saturday temperatures in Usk in nearby Monmouthshire reached 29.2C (84.5F).
Sky News' Defence Correspondent Alistair Bunkall said sources had told him that two men had been "affected by conditions", with heat exhaustion thought to be involved.
Police confirmed a third man was taken to hospital and is in a serious condition.
Bunkall said all the men were in the army. He said the MoD is releasing no details about what sort of training they were on.
Thirty members of Central Beacons, Brecon, Western Beacons and Abergavenny-based Longtown Mountain Rescue teams joined the operation near Pen y Fan - the highest mountain in south Wales.
The Brecon Beacons are well known as a training destination for both the regular army and for special forces like the Special Air Service.
The SAS has its regimental headquarters in nearby Hereford and the Army infantry has its Infantry Training Centre in the town of Brecon.
Earlier this year Army Captain Rob Carnegie was found dead on Corn Du mountain. He is thought to have been attempting a 17-40 mile march in freezing weather through the Brecon Beacons as part of a selection process for the Special Air Service.
The mountain range is among the highest in the UK, with peaks like Pen y Fan reaching 886m (2,907ft).
In winter, conditions are regularly gruelling, which army chiefs say mimic the toughest conditions soldiers may have to face.