Its switch after 50 years has been triggered by fears that serving instructors could be put at risk by inquisitive members of the public.

One former instructor said: “The threat of attack, of serving instructors being compromised by someone taking pictures and even the public just waiting to watch the course, has forced a rethink.”

Although much of the selection process takes place in the Sennybridge military training area, where access is restricted, a significant amount occurs in the national park, which is open to the public.

The source said: “If you are walking along one of these routes and suddenly see a bloke running towards you with a rifle across his arms and a Bergen the size of a fridge on his back you are bound to ask questions.

“I can remember taking blokes across one route and past a bench where three out-of-breath old ladies sat enjoying their lunch. As we stormed past I heard one of them say, ‘They’re from a special place.’ If they could suss us out, anyone can.”

The number of recruits using the Brecon Beacons for tests has increased, with up to 170 men on one course. The SAS, its marine counterpart, the Special Boat Service, and the new Special Reconnaissance Regiment all do the same test.

The current four-week endurance course is based on both physical stamina and mental ability. It begins with the “Fan Dance”, a run up Pen-y-Fan, a 2,900ft peak in the Brecons, down the other side and then back over the peak again. “No matter how fit you are, it’s hard work,” one former SAS officer said. “At the end of it your legs feel like bloody jelly.”

In the past, the Fan Dance typically weeded out about half those who volunteered for selection. But members of the new special forces regiments have little choice but to pass, so more soldiers are taking part in the main part of the course, which is a training phase. The new special forces volunteers undergo tests in which they march at speed, or “Tab” — tactical advance to battle — across the Brecons.

The soldiers, who must all have served in another unit before joining the special forces, carry a heavy Bergen on their back, weighing between 36lb and 55lb.

The last week of selection is the test week, the climax of which is a 20-hour 40-mile solo endurance test in full kit known as “the long drag” during which a soldier navigates his way between a series of checkpoints using a map and compass.

A number of men have died of exposure on the winter course, including Mike Kealy, a highly experienced SAS major who was one of the heroes of the battle of Mirbat, in Dhofar, Oman, in July 1972. He re-took the course in early 1979 after his appointment to command an SAS Sabre squadron.

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