Troublesome pigeons are being tricked into believing parts of City Hall are on fire - as staff battle to protect the landmark.

The birds have been perching on ledges at Norwich City Council's headquarters, with their mess damaging the 1930s building.

Officials have previously installed a wire and post system to make it hard for pigeons to roost at the grade two-listed site, in St Peters Street.

But this has failed to deter the pigeons from visiting City Hall and causing 'damage and nuisance'.

Officials are now testing an optical gel designed to scare the birds away.

The trial will last six months and has cost the council �464.

A city council spokesman said: 'The problem of keeping pigeons away, due to the damage and nuisance caused by their poo, is nothing new.

'But trying to find a suitable method of deterrent is not always easy.

'Now a new optical gel that scares pigeons because they see it as fire, but which is not an irritant, is being tested on part of City Hall.

'In a trial of a two-metre ledge on City Hall last year the results were almost instant, with no pigeons perching where the optical gel was placed.

'We will now trial the gel on low level ledges above the registrar's entrance for about six months and if it proves as successful will consider using it on other projects and buildings.'

This is the latest attempt to use the gel following a successful free trial last year.

Pigeons can see ultraviolet light. When they look at the gel, it is thought they see flames.

City Hall is also set to receive a �2.2m revamp of its office space and customer contact centre, to make the building fit for purpose.

The council spokesman added: 'Netting, posts and wires are also often used to prevent pigeons, but these wouldn't be an option for some parts of City Hall as more discreet methods of control are required on a listed building - a post and wire system which cannot be seen from the ground is already in use.

'Many buildings in the city centre use some sort of pigeon deterrent, ranging from plastic or stainless steel spikes to full netting.'

The City Hall gel trials follow attempts by Norwich Castle to fight off feathered invaders damaging the 11th century building.

Castle staff have said the birds' faeces is slippery and has 'disease related conditions', which is preventing workers from reaching drains and guttering.

This has led to water seeping into the medieval building's walls. Plans to install a 'pigeon guarding system', which is a post and wire system, have been submitted to the city council.

Ben Price, Thorpe Hamlet city councillor, said he was familiar with the problems posed by pigeons through his work as a gardener.

The Green Party member added: 'We're very lucky to have a beautiful City Hall and it's really important we try to maintain it as best as we can, but as financially viable as we can.'

Do you have an unusual story? Call Evening News reporter Richard Wheeler on 01603 772474 or email richard.wheeler@archant.co.uk