Former Carphone Warehouse deputy chairman David Ross is to be investigated by the Financial Services Authority for possible breaches of stock market rules, according to reports.

Ross abruptly left the company on Monday as it emerged that he had pledged his£157 million stake in the mobile phones retailer as collateral for personal loans.

According to the Financial Times, the FSA is now investigating whether Ross flouted its listing and disclosure rules by failing to declare it.

It has also emerged that Ross failed to declare stakes in three other companies where he is a director for collateral for loans. The companies are bus operator National Express; storage company Big Yellow; and marine safety equipment maker Cosalt.

The combined value of his stakes pledged as collateral was£206.1 million last night, the FT reported.

As the scandal escalated yesterday, London mayor Boris Johnson came under pressure to dismiss Ross from his post as financial advisor to the 2012 Olympics organising committee.

Carphone Warehouse shares fell 4.3 per cent last night, as investors feared Ross might sell his stake in the retailer.

In an interview with Sky News, Carphone’s founder Charles Dunstone, said: “It is certainly an embarrassment to the company and to him. He had never given me any indication that his shares were pledged until I heard on Friday.”