Thames Water has told investors they will be able to demand a refund if Labour come into power and nationalise the utility after inserting a new clause into to its outstanding bonds.
In a stock market document to the Irish Stock Exchange, the company outlined Labour's policies of renationalising water, among other utilities like gas and electricity, as well as railways.
Thames Water said any "future intervention" from the government could affect the company's ability to meet obligations.
It also referenced a speech by shadow chancellor John McDonnell who said that the water industry would be the first utility to be nationalised under a Labour government.
The party claim it would end the excessive prices and dividends.
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Labour's proposal would see the ownership of water and sewage companies transfer to regional water authorities, while existing shareholders would likely be compensated with bonds.
Thames Water say investors will be able to demand their money back quickly if the state takes over a majority of the firm.