It would take around 40 years and cost over £20 billion to replace over 150-year-old cast iron pipes across London, according to Thames Water.

The revelation came at a meeting of the London Assembly Environment Committee where councillors grilled Thames Water heads on why the capital is seeing a greater number of pipe bursts recently.

It was put to the representatives from Thames Water that their maintenance of their pipes was not up to scratch.

Jennette Arnold, council member, said: “I share the anger and frustration the sense of being let down by Thames Water.

“It seems to me, and I’m speaking on behalf of people I’ve talked to, that your maintenance programme is based on a ‘let it flood, let it burst’ and then we come in and replace that piece of pipe.

“They don’t seem to have any sense of any progress around your maintenance programme in London.”

Thames Water admitted their maintenance was “not good enough” but pointed out that the pipes they were dealing with weren’t made or laid to modern standards.

Richard Aylard, director of external affairs and sustainability from Thames Water, said: “They are 150 years old. They weren’t cast to modern standards of quality control.

“They were laid by men using block and tackle and pulleys from the back of carts. Some of them were dropped on the ground.”

When asked whether it would be possible to replace the old pipes with modern, more durable pipes, Danny Leaman, head of water networks at Thames Water, said: “Our estimate at the moment is that it would probably take somewhere in the order of 40 years and cost somewhere in excess of £20 billion.

“1km of pipe replacement would take somewhere in the order of six to eight months and we have 2,000 km of pipes across London.

“Anywhere in excess of 3 million litres of water in an hour would be expended from these pipes. What we don’t have currently on those pipes is a lot of automation.

“The ability to access these mains remotely, that’s something we’re looking to invest in the future.”

He also explained why it can be so difficult to deal with bursts on the ‘trunk main’ pipes, the largest pipes up to 24 inches wide, that supply most of the water to the smaller distribution pipes across London.

He said: “Trunk mains move water at high speed, with very few connections and usually don’t leak very much.

“When they burst a lot of water comes out. You can fill an Olympic swimming pool in 40 minutes with one of these trunk mains.

“These pipes can provide in excess of 100,000 to 150,000 customers and within that we’ve got hospitals and other significant special needs customers so we need to be very careful if we cut off those water supplies.

“It can take between two to five valve operations [to stop water flowing through a trunk main], and it takes up to 20 minutes to shut these valves.

“Each one takes two people to shut as they are very large. Most of these valves are located in the middle of four way junctions in the middle of London.

However they agreed that they should be doing better on maintenance currently. Mr Aylard said: “The maintenance is obviously not good enough because we have been devastating our customers’ lives.

“We’ve got to do better at it. The question is how to spend money to get the biggest possible impact.”