David Ross, the embattled Carphone Warehouse co-founder, broke his silence yesterday to say that he had volunteered to resign all four of his public company directorships at the beginning of this week.

A spokesman for Mr Ross told The Times: “David offered to resign from the boards of four public companies as a result of an apparent and unintentional technical breach of disclosure rules concerning the use of shares he owns in them.”

Mr Ross parted company with self-storage specialist Big Yellow group yesterday, having quit the boards of Carphone Warehouse and National Express earlier this week. He could have lost all his lucrative City directorships by the end of the week.

He remains a board director of Cosalt but the Grimsby-based family firm bought by Mr Ross's grandfather nearly 60 years ago is also expected to end its association with Mr Ross within days.

This week Mr Ross stepped down from National Express and resigned from the board of the London Organising Committee of the Olympic Games (Locog) and as the chairman of the Legacy Board of Advisers.

The Conservative Party donor declined to comment yesterday on his personal finances. But it is believed that Mr Ross owes JPMorgan, the investment bank, more than £100million, a loan that he secured on his holdings in the four public companies without registering it. JPMorgan could force Mr Ross to sell all his shareholdings, although a source close to the entrepreneur insisted their would be no fire sale.

“David Ross is now focused exclusively on managing his private business interests in a structured manner, particularly through the present period of market turbulence,” Mr Ross's spokesman said. “David regrets the present situation but his record of previous business success speaks for itself and he is determined to successfully work through this period.”

It emerged at the beginning of the week that Mr Ross had failed to declare that he used shares in Carphone Warehouse to secure personal loans. Big Yellow has since been mulling whether to accept his resignation.

Big Yellow said: “David Ross tendered his resignation as a director of the company at the start of this week and, after careful deliberation, the company has accepted the same with immediate effect.”

The board of Cosalt, which makes safety equipment, has been locked in urgent talks over whether Mr Ross could continue as chairman of the company.

Mr Ross holds a 19.4 per cent stake in Carphone Warehouse, the telecoms retailer founded with Charles Dunstone, his school friend, in 1989. He began working for Mr Dunstone's venture as the financial director of Carphone Warehouse in 1991.

It has since emerged that the Financial Services Authority, the City regulator, has started an investigation into Mr Ross's conduct.

In the chilly market climate, buyers for Mr Ross's Carphone Warehouse stake may be thin on the ground.

Mr Dunstone is adamant that it will not be him, not least because by increasing his 32.6 per cent stake he would be forced to buy the company.

“Not only is it implausible, it's physically impossible,” he said. “The only way I could buy a single share from David or anyone else is if I bid for the whole company. Where would I get the money from? There's no money out there. Even at the current share price I don't have the money to do it. And if I did everyone would think it's incredibly fishy if I was to do it anyway.”

However, before the David Ross uproar, when the stock price hit 100p at the beginning of the month, some analysts had started to question if it might make sense for Carphone's co-founder to take the company private.

“There could be a couple of possible exits for him,” one telecoms analyst said. “The telecoms company will be very valuable to someone.”

Times Recommends

• C&G axed - 1,660 jobs to be lost
• UK house prices rise 1.1% as buyers return
• Mixed reaction to US banks repaying bailouts