The murder of a Claygate family in the French Alps may never be solved after key evidence was said to be "accidentally contaminated".

Saad Al-Hilli, 50, of Oaken Lane, his wife Iqbal, 47, and his mother-in-law Suhalia Al-Allaf, 74, were shot dead in their BMW near Lake Annecy on September 5 last year.

A 45-year-old French cyclist, Sylvain Mollier, was also killed.

Eric Maillaud, the Annecy prosecutor leading the inquiry, reportedly admitted that an expert working for a police forensics team “accidentally contaminated” crime-scene material with his own DNA, the Daily Telegraph said.

Mr Maillaud said the expert was examining a ballistics sample, thought to be a spent cartridge or part of the broken gun, but used his bare hands, leaving his DNA on the item.

It was reported the error was only picked up with the sample was sent to a laboratory in Bordeaux.

It is believed DNA samples have now been taken from up to 100 police and emergency workers who attended the scene to ensure their DNA is not confused with the killer’s.

The DNA blunder is not the first encountered by the French authorities.

After the shooting, four-year-old Zeena Al-Hilli was left inside the family’s car for about eight hours as officers failed to spot her hiding underneath her mother’s skirt in the back of the car.

The youngest Al-Hilli child spent the night surrounded by dead bodies before officers arrive the next morning and rescued her.

Zeena was unhurt in the attack and her seven-year-old sister Zainab was believed to have been shot in the shoulder and pistol whipped.

Surrey Police said they are continuing to provide support to the French investigation into the murder of four people near Annecy on September 5 last year.

The police force’s most recent statement, not released in connection to the latest revelation, said: “Following the establishment of the Joint Investigation Team (JIT) on September 21 last year, officers from the Surrey and Sussex Major Crime Team have been working closely with the French authorities to progress the investigation in a bid to bring those responsible for this crime to justice.

“In addition, the force continues to provide ongoing support through family liaison officers for the relatives of the Al-Hilli family in this country.

“This is a complex enquiry and we continue to have a team of officers dedicated to enquiries in the UK which remain ongoing.”