AN inquest into the death of the commanding officer of the Poole-based Special Boat Service following a diving incident in Norway has been concluded in secret for reasons of national security.

Lieutenant Colonel Richard van der Horst, 38, was taking part in Nato exercises near the Olavsven naval base in the Arctic Circle in March when he got into difficulties.

It is thought that some of the 14,000 troops taking part in the exercises were training for retaking oil rigs and ships from terrorists.

Lt Col van der Horst, was at the rear of a six-seater Swimmer Delivery Vehicle, a mini-submarine designed for the US Navy Seals special forces.

Although capable of carrying frogmen 50 miles underwater, the SDV has no air supply of its own, so all those on board have to carry full diving gear with their own oxygen.

It appears that Lt Col van der Horst experienced difficulties with his supply and was unable to follow his men as they left the vessel to swim in the water. They struggled for 10 minutes to free him, but he was unconscious when he was brought to the surface.

Lt Col van der Horst spent a week on life support in North Norway University Hospital, but died without recovering consciousness.

A post mortem examination carried out by Dr Ian Calder, a Cambridge-based expert in diving accidents, found that Lt Col van der Horst died from fluid in the lungs as a result of fluid in the brain, which was damaged by lack of oxygen.

District coroner Sheriff Payne, who recorded a verdict of accidental death, told the Daily Echo he had been instructed to hold the inquest in private. "I had a long letter from the Treasury Solicitor explaining why it would be dangerous if it was held in public. We have held the inquest in camera on grounds of national security."

It is understood that some of the witnesses at the inquest were members of the highly secretive SBS, who would have had to discuss classified equipment in detail.

A Ministry of Defence insider described Lt Col van der Horst as "a great loss, not just personally to his service, but also professionally".

He added: "His experience was extraordinary and it's always difficult to replace people with this sort of background."

Lt Col van der Horst was born in Devon and went to school at Sherborne. He graduated in biological science at Birmingham and followed his father Rupert - who also commanded the SBS from 1978-1980 - into the Royal Marines.

After the first Gulf War, he was involved in policing the Kurdish haven in northern Iraq so that aid agencies could operate without fear of attack.

He was chosen for special forces in 1991 and in 2000 helped plan the successful operation to free six British soldiers kidnapped during UN peacekeeping operations in Sierra Leone.

He subsequently worked as liaison officer with US Central Command during preparations for the second Gulf conflict.

As deputy director of operations to the commander of coalition special forces, he went all the way to Baghdad and was later awarded the OBE for his service.

Lt Col van der Horst took command of the SBS last November and was widely seen as a future candidate for director of special forces.

As CO, he did not have to take an active part in exercises, but wanted to do so to keep in touch with his men.

His funeral took place with full military honours in Milton Abbey.

Lt Col van der Horst left a widow, Belinda, and two daughters.

First published: November 12, 2005