Feathered invaders are threatening the future of Norwich Castle after breaching its defences - with their droppings.

Pigeons roosting at the 11th Century landmark are making so much mess that they have created a health and safety risk.

Officials say the birds' faeces is slippery and has 'disease related conditions', which is preventing workers from reaching drains and guttering so they can be unblocked to stop water seeping into the medieval building's walls.

Proposals to improve the castle's defences with a 'pigeon guarding system' have now been submitted to Norwich City Council.

This involves installing a stainless steel post with thin wire on it to stop pigeons roosting and perching at various locations around the Castle Museum, including ledges and window sills.

Netting will also be used to prevent the birds gaining access and messing up the area again. It is has not yet emerged how much the project would cost.

The pigeon proof plans warn water penetrating the castle museum is 'affecting both the long-term fabric of the building and potentially the artefacts within it'.

It is suggested a trial of the anti-pigeon measures will take place as part of the planning process.

The proposals add: 'The proposed installation of a pigeon guarding system is considered essential to protect a prestigious public building together with the valuable artefacts exhibited within the museum galleries, whilst also providing much improved health and safety measures for visitors, members of staff and maintenance contractors accessing the building.'

Simon Collins, NBC Bird and Pest Solutions area operations manager for Norfolk and Suffolk, said birds of prey, such as harris hawks, were also used to deter pigeons from visiting areas.

He said: 'We are quite an ethical company in our approach and we don't roll up with an air rifle.

'Pigeons have a right to live where they want as much as we do. So we go down the proofing or bird of prey routes, which is to try and push pigeons away from the buildings.

'The pigeon problem is increasing. If people ring up the council and say they have a problem with pigeons, the council distances itself and says you will have to get independent pest control.'

Norwich City Council says it uses a wire system at City Hall similar to the one proposed at Norwich Castle.

The authority says it was installed in 2010 and has proved to be �6,500 well spent.

A spokesman said: 'We used to have loads of pigeons at City Hall and now there are no pigeons on there - it works.'

Glynn Burrows, owner of Norfolk Tours, said it was vital to protect the castle as it was one of the city's most important buildings.

He said: 'The castle is one of the things I take people to see even if they don't think they want to go and see it.

'A lot of the people I have come over have not heard of Norwich so they leave it up to me for where to take them.

'I normally take them to the back streets to show how many churches the city has. You turn the corner, come out a shopping centre and have a castle - the keep is so fantastic.'

Norwich Castle is not the only eastern landmark attempting to protect its walls from the destructive effects of water.

Lincolnshire County Council officials are currently spending around �4m on repairing and water proofing Lincoln Castle's wall walks.

Cracks have appeared at the castle, built in 1068, due to water inside the walls freezing and thawing throughout the winter months.

What do you think the council should do? Write to Evening News letters at Prospect House, Rouen Road, Norwich, NR1 1RE, or email eveningnewsletters@archant.co.uk