Frederick Barclay's nephews sold the Ritz hotel for "half the market price" after secretly recording conversations between the billionaire co-owner and a Saudi investor offering £1.3bn (€1.5bn) for the London landmark, the High Court has heard.
The 85-year-old businessman and his daughter Amanda are suing three of his twin brother David's sons - Alistair, Aidan and Howard - and Aidan's son Andrew over 94 hours of recordings made over several months.
The High Court has previously heard that the "elaborate system of covert recording" came to light only in January when Alistair was filmed on CCTV "handling the bug placed in the conservatory at the Ritz".
At a further preliminary hearing, conducted remotely yesterday, Mr Justice Warby heard that the recording of Frederick and Amanda Barclay's conversations was "commercial espionage on a vast scale".
Hefin Rees QC, representing the pair, said in written submissions to the court that the recordings "captured over 1,000 separate conversations over a period of months", including conversations with the claimants' lawyers as well as "bankers and business people".
The court was told that Frederick, who had "placed great trust" in Aidan and Howard Barclay to run his and his brother's business empire, is "now left to contemplate his nephews' betrayal".
Frederick and Amanda Barclay are bringing a legal action alleging misuse of private information, breach of confidence and breach of data protection laws, against their four relatives and Philip Peters, who "holds a board position" in the Barclay group of businesses.
Mr Rees submitted that material previously disclosed to his clients "reveals beyond doubt that the defendants derived significant financial and commercial advantage from the unlawful use of the recordings".
In March, Frederick said he had received competing bids to buy the Ritz for more than £1bn (€1.15bn), adding that any move to sell the establishment for "below the proper value would give rise to further litigation".
The Ritz has since been sold to an unnamed Qatari investor for an unknown price, reported to be around £750m (€860m).
Mr Rees told the court the defendants heard "Frederick's conversations with Sidra Capital, which at the time had made an initial offer of some £1.3bn for the acquisition of the Ritz hotel".
He added: "Despite this, the defendants sold the Ritz hotel to another buyer from Qatar at a price that appears to be for half the market price. One is left to speculate why."