Anglian Water first announced the company was on target to lift the notice for up to 50 per cent of the 108,000 homes which were last week told to boil their water.

But yesterday it said this figure had been revised upwards, to more than 60 per cent of customers.

The Pitsford Water Treatment Works has been producing water free from cryptosporidium for the past four days.

More than 500 employees and contractors are still working round the clock to get the water supply back to normal.

The company is in the process of cleaning and purging the water distribution.

Peter Simpson, Anglian Water's chief operating officer, said: "We have identified the cause and are well into the recovery phase.

"Our focus is now to ensure that any final traces of cryptosporidium are removed from the water already in our distribution network.

"Once we are absolutely satisfied that this has been achieved, we will move as quickly as possible to remove the current restrictions, subject to the agreement of the health authority and local authorities."

The boil notice will remain in place while the water distribution network is flushed and cleaned and quality checks carried out.

Anglian Water said customers must continue to boil drinking water until they receive a notification card.

The boil notice will be lifted for the remainder of customers over the subsequent fortnight.

Schools, hospitals and vulnerable customers have been supplied with water, with some 500,000 litres of bottled water being delivered at the rate of about 95,000 litres a day.

The source of the cryptosporidium at the treatment works was identified and isolated in the early hours of Friday.

Anglian Water said the Drinking Water Inspectorate would carry out an investigation.