Carl Icahn, the billionaire Wall Street investor, has raised his stake in Motorola, the troubled US mobile phone maker to 7.6 per cent.

In a filing made on Wednesday with the Securities and Exchange Commission, Mr Icahn reported that he now owns 172.2m Motorola shares, up from 144.6m or 6.4 per cent previously and has spent a total of $2.36bn acquiring the stake.