Water pipes stretching more than 35 miles around Brighton and Hove will be replaced as part of a £15.5 million overhaul.

Southern Water yesterday announced plans to restore the city's network, which is between 120 and 150 years old.

The work is expected to take three-and-a-half years and the company says the revamp is essential.

It claims replacing the 57km of old metal pipes with plastic ones will reduce the number of bursts and leaks as the country experiences its first drought order in 11 years.

But residents and businesses affected will lose their water supply for up to 12 hours and work in each suburb will take between six and eight weeks to complete.

However, Southern Water says it will not start seafront roadworks until the autumn or winter so as not to affect the tourism trade.

Sharon Thomas, chair of the North Laine Traders' Association and owner of The Off Beat Coffee Bar, Sydney Street, Brighton, said: "It will be horrendous. There's not going to be anything good about it, that's for sure.

"We don't know when it is going to happen but we have asked if certain periods could be avoided, such as summer and pre-Christmas shopping.

"We will no doubt lose thousands of pounds in trade. I don't think there is any talk of compensation.

"Most shops in North Laine are small independent businesses and don't have huge resources to fall back on. We rely on day-to-day trade to survive and we rely on locals and tourists being able to get into the city easily.

"We know this has to be done; we all need water for our businesses. I just hope they will carry this out as quickly and efficiently as possible with consideration for the small businesses."

Mike Dodd is chair of St Peter's Traders' Association and owns Japanese restaurant Okinami in Church Road, Hove, and Japanese shop Okinami in York Place, Brighton.

He said: "This work will have to be done if we don't want to keep losing water, but 57km of roadworks, that will bring us to a standstill.

"They should have done this work ten years ago, then we wouldn't have had any of these shortages. I imagine this means bills will go up.

"All the roadworks do not make Brighton an attractive destination. Every time there are roadworks, businesses lose trade. We have had very little notice about this."

But Southern Water, which is investing more than £250 million in schemes to improve water supplies during the next five years, says the work will minimise future traffic disruptions.

Lawrence Gosden, utility services manager, said: "The company carries out regular maintenance work on the pipes but now they are starting to show signs of their age.

"We need to replace the old metal water pipes in Brighton and Hove with stronger plastic ones that will reduce the number of pipe bursts and interruptions to supply and save water by removing any leaking pipes."

The scheme is scheduled to begin this June and finish by the end of 2009. It will be carried out by Clancy Docwra on behalf of Southern Water.

The work will take place across the city, stretching along the seafront from the King Alfred Centre, Hove, to Brighton's Palace Pier, and up through the city centre.

A time line of works has not yet been released but the company will work in three zones - eastern, western and central - with sections of each starting on June 5.

Outdated fire hydrants will also be replaced during the scheme and better positioned. This will be the first time a fire authority will have the opportunity to install its own fire protection network.

Mark Froud, chief executive at Sussex Enterprise, said: "The business community needs reassurance from Southern Water that this on-going work will be planned to cause the minimum amount of disruption to the city's transport system.

"Time spent in traffic queues is unproductive and if visitors are deterred from coming to Brighton because of congestion, the impact on the local economy could be devastating.

"Businesses and residents need to be kept fully informed of exactly where and when this work will be carried out and how any major problems arising from traffic congestion will be dealt with."

A spokeswoman for Southern Water said it was working closely with Brighton and Hove City Council and, where possible, co-ordinating works with other utilities to ensure disruptions were kept to a minimum.