A property tycoon's plans for a mega-basement to use as a museum for his collection of vintage cars have been condemned by the French government as a threat to the "integrity and security" of the French embassy in London.
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.
They are locked in a High Court battle with French lawyers, who are arguing the "over-sized" project is legally flawed and should be blocked.
Mr Justice Holgate, sitting in London, said at the end of a two-day hearing he would give his judgment as soon as possible.
As lawyers left court, the French government warned: "The over-sized proposed development at No 10 Kensington Palace Gardens is threatening directly the integrity and the security of our building in contravention of the Vienna Convention on Diplomatic Relations 1961."
Alexandre Terrasse, of law firm Gordon Dadds LLP, issued a statement on behalf of the French at the doors of the court, saying: "Our property is a place of work and it is used extensively to receive officials from all over the world."
Mr Terrasse added: "The French embassy in London is not opposing a development at No 10 KPG provided that it is within the remit of what is acceptable and reasonable in the circumstances."
The court hearing involved complex issues of planning law. The project was first granted planning permission and listed building consent in 2008, and a smaller scheme was given listed building consent in 2010.
In 2011, the Crown Estates, freehold owners of No 10 KPG and the French embassy building, granted a limited licence to allow certain works of excavation to go ahead.
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 basement.
W Robert Griffiths QC, appearing for the French authorities, argued the listed building and lawful development certificates were both flawed for a number of reasons and should be quashed.
Mr Griffiths told Mr Justice Holgate there had been lack of consultation, and the excavation works already carried out did not amount to commencement of development but were works of repair and refurbishment under the Hunts' lease.
If the French government win their case, it could lead to the Hunts having to make fresh applications to finish the work - under stricter conditions now being imposed for basement developments by the local planning authority.
Paul Brown QC, appearing for the Hunts, told the judge any technical "breach" of the regulations had now been cured and there was no genuine case against them.
He said of the Hunts: "They are acting lawfully. They have permission."
The French authorities had taken legal action because "they don't want (Mr Hunt) to complete the basement," said Mr Brown.
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.