Businessman Patrick McKillen feared that the property tycoons, Sir David and Sir Frederick Barclay, were hacking his phone, he has told the High Court in London.
The Belfast property developer said he had "no evidence" but told Mr Justice David Richards that he was "very concerned".
"I was concerned that the Barclays were hacking my phone," he said. "They do use detectives. I know they had detectives following me into restaurants. There's nothing these people would not do. Putting detectives on me. Using my American social security number, which is illegal."
Kenneth MacLean QC, for companies owned by the Barclays, said Mr McKillen had "no evidence to put before the court" supporting the hacking allegation.
Mr McKillen is embroiled in a battle over control of three of London's leading hotels - Claridge's, the Connaught and the Berkeley.
His claims emerged as he was giving evidence during a trial which started after a boardroom fight over control of a company which owns three London hotels spilled into the High Court.