The French government has lost a High Court action to block a rich man's bid to complete the building of a mega-basement he intends to turn into a museum for his collection of vintage cars.

Jon Hunt, the billionaire founder of Foxtons estate agents, and his wife Lois have already started work at their Grade II listed mansion at 10 Kensington Palace Gardens - the former Soviet Mission next door to the French embassy in London.

They were first granted planning permission and listed building consent in 2008, and a smaller scheme was given listed building consent in 2010.

In 2011, the freehold owners Crown Estates granted a limited licence to allow certain works of excavation to go ahead.

At a recent hearing before a judge at the High Court, the French government challenged the validity of certificates of lawfulness granted by the Royal Borough of Kensington and Chelsea last April to allow completion of the property tycoon's project.

Mr Robert Griffiths QC, appearing for the French authorities, had argued that the listed building and lawful development certificates were both flawed for a number of reasons and should be quashed.

But today Mr Justice Holgate, sitting in London, announced that he had dismissed the application to quash the certificates.

Paul Brown QC, for the Hunts, told the judge at the hearing of the case earlier this month: " They are acting lawfully. They have permission."

The French authorities had taken legal action, he said, because "they don't want (Mr Hunt) to complete the basement".

In 2007 Mr Hunt sold the Foxtons estate agencies chain for £370 million just weeks before the credit crunch hit and the property market slumped.