The new WHO standard, at one-fifth of the existing limit, says that drinking water should contain no more than 10 micrograms of lead per litre.
Tap water is the main source of exposure to lead in the environment - mostly from household plumbing. Areas of Britain with soft water, such as Scotland, Wales and northern England, have a particularly bad record since their water dissolves lead from pipes easily.
It is estimated that the average lead concentration of water in around 4 million homes in England alone would regularly exceed the new limit.
However, a spokesman for the Department of the Environment said that it did not intend to review the implications of the new guidelines until early next year, when they are published formally. Britain's limit will stay at 50 micrograms of lead per litre.
Andrew Lees, campaigns director of Friends of the Earth, said the delay was 'a bureaucratic nonsense - particularly when children's health is at risk'.
American research published 10 years ago and now widely accepted by scientists showed that levels of lead found in children's bodies in some parts of Britain harmed intellectual development.
Water authorities in some areas have been removing their own lead piping and adding chemicals to water to reduce the amount of lead it dissolves from pipes. But they argue that they are not liable for lead that dissolves in tap water once it leaves the network and crosses into private property.
Specialists at the Water Research Centre in Oxfordshire have said the new limit will be extremely difficult to meet without taking out all remaining lead pipes.
Some companies offer limited lead replacement grants, but Mr Lees said that this was not enough. 'The Government must change the grant scheme to ensure that everybody can afford to rip out lead pipes without delay.'
The campaigning organisation says ministers have been reluctant to admit there is a problem because of the cost of replacing pipes. The Government's Drinking Water Inspectorate estimates that the total cost of replacing all existing lead piping would be a maximum of pounds 2.6bn for the water supply industry and a further pounds 5.9bn to individual households.
The WHO convened a meeting of 34 countries in Geneva last week to review its drinking water guidelines. In a statement issued yesterday it said that prolonged exposure to lead 'may lead to serious neurological damage, especially among infants, children and pregnant women'.
In 1989, a report from the Ministry of Agriculture concluded that 'for bottle-fed infants, average water lead concentrations should not exceed 10-15 micrograms per litre'. Mr Lees said yesterday: 'The water companies must warn their customers about the health risks posed by lead in drinking water. Leaflets should be included with the water bills.'Reuse content