Carl Icahn, the billionaire Wall Street investor, has raised his stake in Motorola, the struggling US mobile phone maker, to 6.3 per cent from a previously reported 5 per cent.

The increase in the stake, disclosed on Wednesday in a filing with the US Securities and Exchange Commission, comes as Mr Icahn steps up pressure on the beleaguered Motorola board and management led by Greg Brown, chief executive, ahead of the company’s annual meeting on May 5.