Churches and charities are being charged for the rain on their roofs
Hundreds of churches and charities across England have been hit by what they call "sky-high" increases in water company charges.
Campaigners say churches may cut back on good works or even close down because they cannot afford the increases - of 1,300% in some cases.
Thousands of protesters have signed a petition against the charges on the Downing Street website.
But water firms say the rises are in line with official advice.
Church of England campaigner Martin Dales described the charges as a "rainwater tax". In some cases, he said, parishes had seen water bills rocket from £80 to £800 and more.
He told the BBC: "It's unjust, unfair, and means that money that should be spent on things like children's creches and lunches for pensioners is now being earmarked for the water bill."
Some churches, he added, faced closing down altogether.
One parish considering that option is North Thornaby in Yorkshire.
Churchwarden David Boddy said he and his fellow parochial church councillors were shocked when they received a £800 drainage demand from Northumbria water.
He estimated that it was a 1,300% increase on the previous year.
"We currently have two churches in our parish and this increase means we are seriously thinking of closing one of them to save money," he told the BBC.
Mr Boddy was so incensed that he started the petition on the Downing Street website.
By Friday, nearly 40,000 people had added their names - making it one of the most popular petitions on the site.
The signatories want Gordon Brown to force water companies to exempt churches and charities from the higher charges.
The bills are rising because some water companies now charge churches the full cost of draining the rain that falls on their roofs.
In the past, many churches were exempt because drainage charges were based on their "rateable value" - often calculated as close to zero.
But now the bills are levied according to the total surface area of roofs and other hard surfaces - such as pavements and car parks.
It means that in many cases churches find themselves being charged substantially more than shops and other profit-making premises.
Water companies argue that churches and charities need to pay their share of the cost of draining rainwater away into the sewers.
But the Church of England says they should be exempt because they provide a social service, which is now being put at risk.
It estimates the new charging policy has landed it with a total water bill increase of up to £15m a year.
Other not-for-profit organisations like the Scout Association are also being hit by the new charges.
Scout spokesman Simon Carter said: "We have hundreds of scout groups who own their own buildings.
"They face a choice: do they organise activities to keep kids off the streets, or do they pay their water bills?"
So far four English water firms - Northumbria, Severn Trent, United Utilities and Yorkshire Water - have imposed the charges.
Campaigners fear England's other water companies will soon follow suit.
In Scotland, small charities, churches and voluntary groups have been given exemption from water charges for the next six years.
Welsh Water said it had no plans to change its charging system.
A protest against "unjust" drainage bills is set to be launched soon.
And as well as lobbying MPs, the Church of England is due to hold a special debate on the issue at next month's General Synod meeting.
Water companies say they are only following the advice of the watchdog Ofwat, which maintains that charging for drainage costs is "the fairest way."
A spokeswoman for Water UK, which represents the firms, said: "All companies are more than willing to listen to customers affected by the new billing arrangements who may have difficulty in paying their bills within the normal timeframe."
An Ofwat spokesman said: "Charging for drainage is an environmentally responsible approach and forces organisations to take responsibility for the drainage of their properties."
Ofwat says the cost providing surface water drainage in England and Wales is about £700m each year.
It says some not-for-profit organisations - such as charity shops - have seen their water bills fall as a result of the new charges.
However, the protests have prompted a rethink by the government - which up until now has backed the policy.
A spokeswoman for the Department for the Environment told the BBC: "Ministers are currently reviewing the situation for groups such as churches and charities."