David Ross, the disgraced co-founder of Carphone Warehouse, has tendered his fourth resignation in as many days, this time as director of Big Yellow Group, the self-storage specialist.

It has taken Big Yellow since Monday to decide whether to accept Mr Ross's resignation who began the week by failing to declare using shares in Carphone Warehouse to secure personal loans.

Cosalt, the Grimsby-based family firm bought by David Ross’s grandfather nearly 60 years ago, is also expected to end its association with the Conservative Party donor within days.

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 has already resigned from the boards of Carphone Warehouse and National Express and as Boris Johnson’s senior Olympics adviser this week. The Carphone Warehouse co-founder could have lost all his lucrative City directorships by the end of the week.

Big Yellow said this morning: "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."

Mr Ross holds a 19.4 per cent stake in Carphone Warehouse, the telecoms retailer founded Charles Dunstone, his schoolfriend, in 1989. He joined Mr Dunstone's venture as the financial director of Carphone Warehouse in 1991.

As well as using 15 per cent of his Carphone Warehouse shares to secure personal loans, Mr Ross used his holdings in Big Yellow, National Express, where he was chairman, and Cosalt, the marine safety equipment group specialist he also chairs, as collateral without disclosing the transactions.

In total, Mr Ross mortgaged his entire £200 million portfolio of shares across the four companies without informing any of the companies concerned.

It is believed he used the loans to support his property holdings, which include Nevill Holt, the former home of the Cunard family in Leicestershire which plays host the the Grange Park Opera every summer.

He also owns a home in Knightsbridge, a villa on Mustique, and Manor Farm, a 1500-acre estate in Northamptonshire.

Earlier this week, Mr Ross stepped down from National Express and also resigned from the board of the London Organising Committee of the Olympic Games (LOCOG) and as the chair of the Legacy Board of Advisers.

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

The regulator declines to comment; however, as the primary regulator of the London Stock Exchange, it must examine all possible breaches of market rules.

Mr Ross's grandfather John Carl Ross bought Cosalt after the second world war with money from the family trawling operation, now part of Young’s seafood. He was chairman of Cosalt when the company floated in 1971 and his son John, David’s father, was chief executive until he resigned in November 1986 over a fall in profits. It is based in Grimsby, where David Ross grew up.

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