Water firms Thames Water and United Utilities have failed to meet targets for cutting leaks, Ofwat has said, amid warnings of water restrictions.
More hosepipe bans are expected in southern England
Thames loses 915 million litres a day, the regulator said - enough to fill 366 Olympic-sized swimming pools.
The firm, which has missed targets for a third year running, has warned it may bring in a hosepipe ban by mid-August.
The UK's 23 water firms have managed to reduce overall leakage by about 1%, or 40 million litres, Ofwat said.
The news comes a day after the Environment Agency called on water firms to restrict water use "sooner rather than later" to avoid worse problems later in the summer.
Thames Water failed to reduce its leaks to a target of 905 million per day and, in its area, nearly a third of the water leaks away before it reaches customers, BBC business correspondent Nils Blythe said.
The company says it is in the early stages of a £500m programme to fix the leakiest pipes in its supply network.
Ofwat's director general Philip Fletcher was scathing about Thames Water's performance, saying the "only bright side" was that the firm had reduced leakage this year by a "significant" 3% or 30 million litres per day.
The regulator also set the group a new target of reducing leakage to 725m a day by 2009-2010.
Meanwhile, despite United Utilities being set a target of 470 million litres of leakage, 500 million litres of water has been disappearing from its pipes daily.
In April, Ofwat was awarded new powers to fine water companies who missed their leakage targets.
However, as both Thames and United Utilities - which supplies the north west of England - have been working toward improving their networks to cut leaks - no action is expected to be taken.
Cambridge Water also missed its yearly target, but no action was taken as the firm remained within the three year rolling average used as a guide by Ofwat.
DAILY UK WATER LEAKAGE*
Overall - 3,609 million litres - down 41 million litres
Thames Water - losing 915 million litres, down by 30 million litres. Target 905 million litres
United Utilities - losing 500 million litres. Target 470 million litres
Cambridge Water - losing 14.2 million litres. Target 14 million litres
Barry Clark - of industry body Water UK - said firms had managed to cut leaks by a third since 1995 or enough to supply 10 million households.
"We are on the case in a big way but the industry has had to cover investment in other areas" such as water quality and purity, he told BBC Radio Five Live.
He added that £1bn is to be invested in fixing leaks in coming years, with the aim of cutting leaks by another 300 million litres per day by 2010.
Hampshire, Sussex, Kent and the Thames Valley have only had about two thirds of their average rainfall since November 2004.
Thames Water is starting to use its contingency reserve in north London, the first time since 1997.