Former world rally champion Colin McRae did not have a valid flying licence when the helicopter he was piloting crashed in a wooded valley killing all four people on board, an official accident report has said.

Among those who died was six-year-old Ben Porcelli whose parents said that the accident was "completely avoidable".

In trying to fly in a valley at relatively low height and high speed, Mr McRae, 39, who had driven for years for Banbury's Subaru World Rally Team, was "undertaking a demanding manoeuvre", the report from the Air Accidents Investigation Branch (AAIB) said.

The report added that a phenomenon that can give an impression that the controls are jammed may have led to the significant deviation in the flight path that Mr McRae was trying to recover from. Disorientation, misjudgment or other factors may have led to this deviation but "the cause of the accident was not positively determined", the AAIB said.

As well as claiming the lives of Mr McRae and Ben, the crash also killed Mr McRae's five-year-old son Johnny and 37-year-old friend Graeme Duncan.

The Eurocopter Squirrel helicopter went down on the afternoon of September 15 2007 just 150 yards from the intended landing spot in the grounds of Mr McRae's home near Lanark in Scotland.

In a statement Mark and Karen Porcelli said: "We are relieved that the AAIB report has finally been published, particularly since the circumstances of the accident which took Ben's life were presented to us by the AAIB in February 2008. The cause of the crash is clearly outlined in the report. Most of the flight was captured on video and it is clearly evident that unnecessary risks were taken and that the accident was completely avoidable."

The report said that Mr McRae's five-year flying licence had expired in February 2005 and he was also not authorised to fly the type of helicopter he was operating - his "valid type rating" having expired in March 2007.

The AAIB said that there had been several cases between 2004 and the time of the accident of "non-compliance with existing regulations".

The report added that when Mr McRae had flown from Scotland to London in March 2006 he would have known his type rating had expired since the purpose of the flight was to meet with an examiner to renew it.