A report into the accident, which happened in July 2013, said the gardener may not have been trapped underwater if he had used bar.

Eastern Daily Press: The grassy slope where the accident occurred.The grassy slope where the accident occurred. (Image: Archant)

Lee Abel was cutting the grass with a ride-on mower when it plunged into the Lower Lake, near the estate offices, on July 8.

The machine came to rest upside-down on top of Mr Abel, who lives on the estate and is in his 30s.

Two police officers and Martin Woods, Sandringham's head gardener, went into the water and dragged the mower back onto the bank.

Mr Abel, who was strapped into the driver's seat, was airlifted to Addenbrookes Hospital. His condition was described as critical in the hours after the the accident but he has since made a full recovery.

The accident was investigated by the Health and Safety Executive, which said it found no 'health and safety management issues'.

A spokesman said: 'Our investigation found no evidence of breaches of health and safety regulations, and the case is now closed.'

But the HSE's full 12-page report was not made public. Now the document, marked 'restricted', has been released under the Freedom of Information Act.

The document states Mr Abel was found 'trapped underneath the mower in the lake still wearing his seat belt'.

It concludes the machine slid down a grass bank towards the lake after its rear wheels lost traction.

'The mower than toppled backwards into the water, twisting towards its offside before ending upside down, trapping [Mr Abel].

'The roll bar was in the lowered position and [Mr Abel] was wearing the seatbelt. He was therefore trapped underneath the upturned mower.

'It is not clear why the roll bar was not erected. If it had been in use, he may have been able to release himself from the seat and escaped.'

The HSE also found risk assessments were out of date. It recommended the estate assess the risks of lone-working.

A spokesman for the estate said: 'Sandringham Estate is aware of the HSE report. Any actions relating to their recommendations and observations will be assessed directly with that body.'

The drama happened half an hour after the Queen attended a private engagement in a nearby garden.

PCs Keith Hunt and Darren Wynne, and Mr Woods, are to receive Royal Humane Society testimonials on parchment and the two police are additionally to receive resuscitation certificates for their fight to revive him.

Announcing the awards Dick Wilkinson, secretary of the Royal Humane Society, said: 'It is little short of a miracle that the victim is alive today.

'It was a horrific incident and but for the refusal of the police to give up their efforts to revive him after he had been dragged from the water he would never have survived.'

Mr Wilkinson said Mr Abel was under water for ten minutes before rescuers arrived.