Almost 100ft long, the aviary – believed to be the biggest of its kind in Britain – was officially opened at the RSPCA's East Winch Wildlife Centre, near King's Lynn, today.

It even boasts CCTV cameras which can enable vets to watch birds' flight in slow motion, allowing them to check that damaged wings are on the mend.

'We are over the moon that we now have this fantastic facility at our centre which will have an incredible impact on the work we do with birds of prey,' said centre manager Alison Charles.

'At the moment we have to use a potato barn to give birds like marsh harriers enough exercise before release.

'Now, thanks to this amazing donation from the Katherine Martin Charitable Trust, we can fly the birds in safety at the centre.'

Mrs Charles said that between 1992, when the centre opened, and the end of 2010, it had cared for more than 4,700 birds of prey.

Before they can be released back into the wild, staff must ensure that the birds are capable of strong, straight flight or they could starve through being unable to hunt for their food.

'Clearly this aviary will help to save the lives of many birds,' said Mrs Charles.

'It is very exciting to finally have this aviary.

'The plans have been in the pipeline for almost a decade, but now, thanks to the generosity of the trust, this dream has become a reality.'

The Katherine Martin Charitable Trust, which is based in Linconshire, has previously provided electricity to heat outdoor hedgehog pens at East Winch.

Martin Gardiner, chairman of the trustees, said: 'The wildlife centre is a fantastic facility and the staff do a terrific job.

'We're very pleased that we have been able to support their vital work.'

Centre staff have not revealed the cost of the aviary, which was shipped from America in sections before being assembled on the site.

Open to the elements with slats and louvres at either end, the building is made from a vinyl skin stretched over a metal frame.

From a distance, it resembles one of the Nissen huts which were once a common sight on former airbases across the Norfolk countryside.