One of the world's largest tree houses opened to the public for the first time today.

The huge construction, which is turreted and stands up to 60ft off the ground, has been built in the grounds of Alnwick Castle, Northumberland - used for the filming of the Harry Potter movies.

The Duchess of Northumberland has spurred on the project, which is part of the redevelopment of the castle's gardens into a major visitor attraction.

The tree house, which is the largest in Europe, has room for 300 visitors and has been built around suspended walkways which will be accessible to wheelchair users.

It is a maze of hobbit-like buildings covering an area larger than the size of two Olympic swimming pools.

The tree house has been constructed around 16 mature lime trees, which will continue to grow unconstrained by the structure and will be linked by 4,000 square feet of suspended satellite walkways ''floating'' in the trees.

It has been designed to withstand all weathers and gale-force winds and has been built from natural materials including Canadian cedar, Siberian larch, Scandinavian redwood and English and Scots pine.

The tree house holds a reception area, shop, cafe, resource rooms and toilets, while giving visitors a tree-top view.

The Duchess, who is a trustee of The Alnwick Garden Trust, hopes it will allow children to play ''dangerously'' while in a safe environment. At the base of the tree house is a huge play area, open to wheelchair users as well as able-bodied children.

The Duchess, who was surrounded by local schoolchildren as she officially opened the tree house, said today: ''This incredible tree house shows that the Alnwick Garden is more than a garden.

''Access and activity for those with disabilities was also at the heart of our tree house design.

''Regardless of their ability, children should be encouraged to get outdoors and appreciate nature.

''Many children now have less opportunity to play than any other generation. We have wonderful plans to expand our 'safely dangerous' play facility.

''The idea is to provide a challenge, and include elements of risk so that you help children better equip themselves for the future.

''And why shouldn't the less able-bodied of all ages go high up and see life up on the trees?''

The completion of the tree house means that the Alnwick Garden is now halfway to completion.

Other developments planned for the future include the controversial Poison Garden - featuring the cannabis and coca plants - which is likely to open later this year.