Nationalising the water industry could cost taxpayers up to £90 billion, a cross-party think tank has said.
Estimating the figure at the request of companies including United Utilities, Anglian Water, Severn Trent and South West Water, the Social Market Foundation (SMF) said their calculation has been published independently.
Their report found that a government which chose to buy the English water industry at fair market prices would pay between £87 and £90 billion to acquire water firms currently owned by shareholders and investors, including pension funds.
Funding such purchases with borrowed cash would add 5% to the national debt, the think tank warned.
The SMF said their calculation is a takeover price based on an assessment of company accounts, and follows valuation methods used by investment banks and fund managers.
Considering options for nationalisation where a government forced through a sale for lower prices, the think tank said this would reduce upfront costs to taxpayers.
But the SMF said it would cost the UK economy money in the longer term as investors in other sectors either deserted Britain or demanded a risk premium to invest here.
The chief economist at the SMF, Scott Corfe, said their research shows "there is no free lunch on offer" and that however it was done, nationalising the water industry would "impose significant costs" on the state and taxpayers.
The report also found that buying water companies below their takeover price would leave current owners out of pocket - including UK pension funds, which are significant investors in the water industry and water industry workers.
In taking ownership of the water industry, a government would acquire currently profitable companies, with future profits weighed against the immediate purchase price, the SMF said.
But the think tank said these future profits would depend on investment in water piping and other infrastructure - costs that would then fall to the government.
Calculating how meeting the industry's current plans would involve spending more than £100 billion by 2040, the SMF said this would mean the Government would spend more than 13% of its entire capital budget on water infrastructure.
The report also noted how political pressure to spend more in areas such as healthcare, education and transport - as well as ongoing strains on the public finances - would mean there is a potential risk the nationalised water industry would not receive the capital spending it required to stay profitable.
SMF director James Kirkup said: "Taking ownership of water companies would mean taxpayers owned firms that are currently profitable, but maintaining those profits would require significant sums of public money to be spent on capital investment.
"Would ministers thinking about re-election really choose to spend billions of pounds of scarce public money on water infrastructure, or would they divert the money to other priorities such as the NHS?
"This study makes no comment whether the policy of water nationalisation is a good or bad idea, but simply sets out the likely costs of that policy.
"Equipped with this information, it is up to voters and politicians to decide whether those costs are a price worth paying."