David Ross has resigned as chairman of Birmingham-based National Express, 24 hours after quitting Carphone Warehouse after it emerged he had failed to declare the use of £120 million of shares as collateral against personal loans.

The co-founder of one of the country’s most successful mobile phone retailers also stepped down from all his high profile roles linked to preparations for the 2012 London Olympics.

Mr Ross, 43, quit Carphone Warehouse on Monday after it emerged that he failed to tell the company, and the stock market, about the mortgaging of his shares.

The shares, part of his 20 per cent stake in the firm, were pledged against separate property investments.

National Express announced Mr Ross had also resigned as director and chairman of the company.

A spokesman said: “The company announces that David Ross has today tendered his resignation as a director and chairman of the board of the company.

“The company has accepted this resignation.”

Tim Score, a non-executive director, has temporarily taken over as chairman of the board, the spokesman said.

Chief executive Richard Bowker said: “We would like to thank David for all of his efforts on behalf of National Express, as both a non-executive director and chairman of the board, since he joined in 2001.

“He has made a significant and positive contribution to our business over this time and we wish him well.”

Mr Ross yesterday stepped down, amid mounting pressure, from his role on the board of the London Organising Committee of the Olympic Games (Locog) and as chair of the Legacy Board of Advisers, in e-mails to Olympics Minister Tessa Jowell and London Mayor Boris Johnson.

Mr Ross, who has donated thousands of pounds to the Conservative Party, also offered to resign from the Olympic Lottery Distributor.

In the email Mr Ross wrote: “I reach this decision with sadness, as I have very much enjoyed making this contribution to British sport, which has been a lifelong passion.

“However, given the present circumstances, and while they are not connected to the Olympics, I must now devote my full attention to my private business interests.

“I also do not wish to distract others from the important work still to do in making 2012 the success I know it will be.”