Thames Water is using a 280-strong team to identify invisible underground water leaks this winter before they escalate into big bursts above ground.

The recent sustained freezing temperatures, which took hold at the end of November 2010, nearly quadrupled the number of leaks and bursts on Thames Water's 20,000-mile network of pipes. The company has been dealing with 700 leaks at any one time, half of which are hidden underground.

Working at night to take advantage of reduced noise levels, the unit uses special leak microphones and "listening sticks" to detect the tell-tale trickles. Other equipment used includes high-tech correlators, GIS maps and more traditional listening sticks, steel rods with wooden or plastic heads through which one can hear sneaky trickles from leaking pipes. Every night leak detectors are sent out to leakage hotspots to pinpoint the exact location of underground leaks.

Kevin Holland from Thames Water said:

"These leaks are much harder to find and fix than the ones which flood roads at the surface, but they often cost us more water and if they aren't dealt with they can lead to bigger bursts with lots of disruption. So we need to locate them as fast as possible.

"Last month was the coldest December for 100 years. This has resulted in a four-fold increase in the number of leaks on our network.

"Although our pipes are more leaky at the moment, pin-pointing these leaks below ground is a difficult job, and takes lots of know-how. We use a range of methods to expose them."

Once detected the leaking pipes need to be pulled out and replaced with new, tough plastic pipes designed to withstand all weather conditions, soil corrosion and traffic vibrations.

When the water in reservoirs drops below 5 degrees Celsius and enters Thames Water's mains, especially the old cast-iron ones, they contract and in some cases break. In recent weeks the temperature of water going from reservoirs in the Thames and London area has fallen as low as 1.8C, the lowest ever recorded at this time of year. The average is 7C.

Kevin Holland added:

"We are facing the prospect of seriously cold water in our reservoirs until Spring 2011, which means lots more bursts and leaks on our cast-iron mains. Although the weather has got milder we're not out of the woods yet. It will take a long time for the water in our reservoirs to warm up."

More than 20 per cent of London's water mains are over 150 years old - the oldest in the UK - and more than 40 per cent are over 100 years old. Replacing the leaking mains is essential as the capital's water resources are under increasing pressure from climate change and population growth. Over the past five years, Thames Water has replaced 1,300 miles of Victorian water mains in the capital, cutting leakage by 27 per cent.

However, Thames' leakage problems are not confined to the winter period. The company is currently working on a six-month, £6m project to reline a major trunk main running through Brentford and Isleworth that burst twice last summer causing major local disruption.. A 4.5km section of the main is being relined with a tough plastic inner-pipe between Twickenham High Street and Brentford football stadium to help prevent future bursts.