Sir Frederick Barclay has released a video which apparently shows one of his nephews handling a bug placed in the conservatory of the Ritz hotel which he claims was part of a “deliberate and premeditated invasion” of his privacy.

The 85-year-old businessman is involved in a bitter High Court battle with three of his twin brother Sir David’s sons over 94 hours’ worth of secret recordings made over a number of months as part of what his lawyers describe as “commercial espionage on a vast scale”.

The footage, which was released on Monday morning, appears to show Sir Frederick’s nephew Alistair just after 11pm on January 13 this year handling a listening device, which is said to have been used to capture more than 1,000 separate conversations.

In a statement, Sir Frederick said: “I do not want anyone else to go through the awful experience of having their personal and private conversations listened to by scores of strangers.

“It is surely in everyone’s interests for the law to be changed to prevent people, outside the authorities, using sophisticated spying devices that have such an intrusive impact.”

He added: “I am putting this video evidence forward as a graphic demonstration of how easy it is to spy on people in public places and to help bring about legislation to prevent such damaging intrusion.”

Sir Frederick and his daughter Amanda are suing Alistair, Aidan and Howard Barclay, Aidan’s son Andrew and Philip Peters – a director of a number of family companies – after the “elaborate system of covert recording” came to light in January.

At a High Court hearing earlier in May, the pair’s lawyers claimed that the Ritz had been sold for “half the market price” after conversations between Sir Frederick and a Saudi investor, who was offering £1.3 billion for the London landmark, were secretly recorded.

Hefin Rees QC, representing Sir Frederick and Amanda, said the recordings “captured over 1,000 separate conversations over a period of months”, including conversations with their lawyers as well as “bankers and business people”.

He said that “a separate wifi bug was also used” which was supplied by private investigations firm Quest, which “invoiced for 405 hours of their time to listen and transcribe the recordings”, with transcripts later “shared amongst the defendants and others”.

The court was told that Sir 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”.

Mr Rees said the defendants heard “Sir Frederick’s conversations with Sidra Capital, which at the time had made an initial offer of some £1.3 billion for the acquisition of the Ritz hotel”.

This is not a case in which private information has been obtained and then widely disseminated to the press – it is not that kind of case at all.

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.”

Mr Rees argued that the recordings allowed the defendants “to anticipate the claimants’ every move in advance, plan their business strategy around that, including knowing what legal advice the claimants were seeking and getting at this crucial time when their business and personal relationships had broken down, and the respective interests of the claimants and the defendants were in conflict”.

Heather Rogers QC, representing all five defendants, accepted that “the claimants have been recorded without their consent or knowledge”, adding: “It is correct that the defendants shared some of the transcripts between themselves.”

But Ms Rogers said: “This is not a case in which private information has been obtained and then widely disseminated to the press – it is not that kind of case at all.”

A formal defence to the High Court claim has not yet been filed and a trial is due to take place at a later date.