A fatal accident inquiry found McRae had been "imprudent" in embarking on demanding manoeuvres in difficult terrain, "contrary to the principles of good airmanship".

Sheriff Nikola Stewart said the racing champ did not hold a valid flying licence and should not have been flying the aircraft at the time.

McRae, 39, his five-year-old son Johnny, the boy's six-year-old friend Ben Porcelli and Graeme Duncan, 36, all died when the aircraft came down near McRae's Jerviswood House home in Lanark on September 15, 2007 as he flew home from a trip to see a friend.

The Eurocopter Squirrel helicopter crashed into trees on the south bank of the Mouse Valley before bursting into flames.

Ben's parents had no knowledge that their son was on the flight and had not been asked for their consent.

The inquiry, which sat over 16 days at Lanark Sheriff Court, has concluded the deaths could have been avoided if McRae had not engaged in unsafe low-level flying.

"It would have been a reasonable precaution to refrain from flying helicopter G-CBHL into Mouse Valley wherein the pilot engaged in low-level flying when it was unnecessary and unsafe for him to do so, and whilst carrying passengers on board," she stated.

The sheriff found there was "no operational or logistical reason" for McRae to have descended into the valley at speed.

"It is difficult to avoid the conclusion that his intention was to conclude the flight as he had started it, with a spectacular and dramatic fly-past of the valley and its vegetation, followed by a steep climb out prior to coming into land, all for the benefit of his passengers," she said.

The sheriff concluded: "The deaths and the accident resulting in the deaths might have been avoided had Mr McRae not flown his helicopter into the Mouse Valley."

She continued: "Mr McRae chose to fly the helicopter into the valley. For a private pilot such as Mr McRae, lacking the necessary training, experience or requirement to do so, embarking upon such demanding, low-level flying in such difficult terrain, was imprudent, unreasonable and contrary to the principles of good airmanship."

McRae's father Jimmy said he hoped the family would be able to "move forward" after four "extremely difficult" years for all the families concerned.

He said: "We will never know what caused the crash but were never in any doubt as to Colin's prowess as a fine pilot - his reactions and eye and hand co-ordination were world-class."

The ruling states that the accident happened when, due to an "unknown occurrence", the aircraft deviated from its path and crashed.

The sheriff said a technical malfunction, interference with the controls, or a bird strike could not be ruled out.