Flying into deteriorating weather conditions, such as those possibly faced in the Gillingham accident, is one of the biggest risks in helicopter flights, according to aviation safety experts.

David Learmount, operations and safety editor of Flight Global, said: 'There have been many incidents when pilots have continued their journey instead of turning back before the conditions worsened.

'What then happens is that the flight continues and the weather gets even worse. Flying into deteriorating weather is one of the biggest risks for helicopter flights, as statistics show.'

The helicopter involved in last night's accident was an AgustaWestland AW139, but exactly what journey was involved is still to be determined.

What will emerge from the investigation by the Air Accidents Investigation Branch (AAIB) team will be the type of flying qualification held by the pilot.

Many helicopter pilots are permitted to fly only in good-visibility conditions during the day, with others being permitted to fly at night in good conditions.

Some pilots are qualified to fly using visual-aid instruments so that they are able to take to the skies when there is cloud, or poor visibility.

When 'flying on instruments', pilots are provided with an artificial horizon so they can keep their aircraft level. They can also make use of compass readings.

Mr Learmount said: 'Helicopter flying is usually done under clear-visual conditions. Helicopters are much more difficult to fly than aeroplanes and much more difficult to fly on instruments than planes.

'You have to be particularly careful if fog is around. It's a hell of a hazard.'

The AAIB will also want to know if there was any communication from the helicopter before or during the flight.

If the helicopter had taken off from a private helipad in good conditions, then the pilot would not have needed any permission to get airborne.

Mr Learmount said the Norfolk accident had echoes of the helicopter crash in south London last year when experienced pilot Pete Barnes, 50, was killed when his aircraft clipped a high-rise crane in Vauxhall.

Pedestrian Matthew Wood, 39, was also killed as he walked to work.

A report by the AAIB revealed the client had suggested to the pilot that he should postpone the journey because of poor visibility.

One of the people killed when a helicopter crashed in thick fog at Gillingham was Northern Ireland peer Lord Ballyedmond, it was confirmed this morning.

Lord Ballyedmond, one of the richest men in Northern Ireland, was chairman of Norbrook, the largest privately owned pharmaceutical company in the world.

He owned Gillingham Hall, a stately home near the crash site.