A mortuary is doubling in size as emergency planners prepare for a possible flu pandemic that could kill up to 4,000 people in the city.

Emergency planners estimate that in a worst case scenario up to half of the population of Brighton and Hove could become infected and up to 2.5 per cent of those could die doubling the city's death toll.

In Brighton and Hove work starts this month on a £250,000 project to increase the capacity of the city's mortuary from 43 to 79.

It is due to become the emergency unit for Sussex. Experts fear the next pandemic could be triggered by a mutation of the deadly bird flu strain. Robin Humphries, Brighton and Hove City Council's emergency planning manager, said the World Health Organisation was monitoring outbreaks of new strains of bird flu closely and warned "we could be talking months" before the flu hits.

Those most at risk would be the elderly, the young and those with respiratory diseases. Mr Humphries told The Argus: "Currently we are at what the WHO calls Phase Three, where it is not spreading yet from human to human. As soon as it does and Phase Four is declared, then we will have only a month or so before it reaches Britain. "We believe that could happen at the earliest any time from this year to 2010.

"As soon as Phase Four is declared, emergency plans will be put into practice including cancelling non-urgent hospital appointments and operations, separating vulnerable patients from flu patients in hospitals and clearing bed blockers from wards." Damien Adams, West Sussex County Council's head of emergency management, said the pandemic could infect a quarter of the population.

On Friday a multi-agency workshop was attended by representatives of GPs, crematoriums, grave diggers, hospital technicians and social services. A spokesman for East Sussex County Council said: "The council is actively preparing for the arrival of a flu pandemic.

"All of its critical services are developing business continuity plans.

"These plans are generic in nature and designed to ensure that each department can continue to provide its key services while facing disruption due to any emergency." GP surgeries are likely to be swamped with patients and the advice is for people to stay away from them and phone NHS Direct instead, where operators are able to prescribe Tamiflu, a drug which alleviates the symptoms of the virus and which the Government is stockpiling. If pharmacies become overwhelmed, distribution centres would be set up in other buildings. One already earmarked for use is Hove Town Hall. In the last 100 years, three flu pandemics have swept the globe, killing millions of people.

After the end of the First World War in 1918, Spanish flu killed up to 50 million people worldwide.

The 1957 Asian flu and 1968 Hong Kong flu pandemics both killed more than four million people.