French tax authorities have had an unexpected windfall after the discovery of thousands of undeclared private swimming pools. 

Following an experiment using artificial intelligence (AI), more than 20,000 hidden pools were discovered which have now yielded €10 million in tax.

Pools usually lead to higher property taxes because they boost house value, and must be declared under French law.

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The software, developed by Google and French consulting firm Capgemini, detected the pools on aerial images of nine French regions during a trial in October 2021.

Tax officials say the aerial surveillance may now be rolled out nationwide.

There were more than 3.2 million private swimming pools in France in 2020, with a pandemic-induced surge in installations.

Le Parisien newspaper reports that an average-sized pool of is taxed at €200 (£170) a year.

The tax authorities say the software could eventually be used to find undeclared home extensions, patios or gazebos, which also play a part in property taxes.

Julien Bayou, of France's Europe-Ecology Greens party, did not rule out a ban on new private pools, claiming that France needs a "different relationship to water". 

Irrigation has been banned in much of the north-west and south-east of France to conserve water and July was the driest month since March 1961.