A major in the British army and a former officer are facing trial by court martial over the deaths of three men who suffered fatal heat illness during an SAS test march in the Brecon Beacons in south Wales.

The two men can only be identified by the ciphers 1A and 1B and sat hidden behind a screen at the court martial centre in Bulford as proceedings began on Monday. Cipher 1A is a major and 1B is a former warrant officer who has now left the armed forces.

Both men deny that they performed a duty negligently, namely “the duty to take reasonable care for the health and safety of candidates taking part in an exercise”.

According to the charge sheet, 1A was the training officer for the exercise and 1B was the chief instructor.

L/Cpl Craig Roberts and L/Cpl Edward Maher were pronounced dead on the mountain range in south Wales after suffering heatstroke in July 2013. Cpl James Dunsby died at Birmingham’s Queen Elizabeth hospital from multiple organ failure more than two weeks later.

Roberts, 24, and Maher and Dunsby, both 31, were part-time soldiers who wanted to try out for the SAS reserve, which works alongside regular troops in some of the most dangerous war zones. The three were taking part in the march on and around Pen y Fan, the highest peak in south Wales, in July 2013. By early afternoon temperatures had reached 30C (86F).

The trial will last around three weeks and current and past members of British special forces may be called to give evidence. The prosecution case will open later this week. The charge carries a maximum sentence of two years in custody.