ONE of the largest tree houses in the world will open to the public today following a £3.3m investment in North-East tourism.

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 massive fun house has 4,000sq ft of suspended wooden walkways, providing an unrivalled experience for British and international visitors.

The tree house has facilities for disabled people and can hold up to 300 people at a time.

It has a wooden cafe, complete with a log fire, and offers scenic views across the Northumberland countryside.

Engineers have designed the tree house to withstand all weathers and gale-force winds - and it has already been severely tested over the past few days.

It has been built from natural materials, including Canadian cedar, Siberian larch, Scandinavian redwood and English and Scots pine.

The tree house is part of the development of the gardens with plans to create one of the best adventure playground's in Europe over the next three years.

The Duchess of Northumberland, Jane Percy, led the project, which is part of the redevelopment of the castle gardens into a major attraction.

She said: "This incredible tree house shows that The Alnwick Garden is more than a garden.

"Many children now have less opportunity to play than any previous generation. Regardless of their ability, children should be encouraged to get outdoors and appreciate nature.

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

"We have wonderful plans to expand our 'safely dangerous' play facilities," she said.

There are plans to invite schools to use plasma screens within one of the tree house rooms to give lessons.

The idea is to bring learning to life with the art of story-telling within the natural environment around them.

Ian Ramage, project manager at The Alnwick Gardens, said: "This will give a tremendous boost to the region's economy during a very exciting and challenging time. We are hoping it will attract people from all over the world.

"It broadens the appeal we have and the idea is to be inclusive. This is about opening up another branch of The Alnwick Gardens to everyone."

He said completion of the tree house meant The Alnwick Gardens project was half finished.

It is being developed and managed by The Alnwick Garden Trust and supported by private and public donors, including The Northern Rock Foundation, Northumberland Strategic Partnership, the European Regional Development Fund and One North-East.

Other attractions including the Poison Garden, Bamboo Labyrinth and an interactive water garden, called the Serpent Garden, which will all be fully planted and operational by the spring