As reported Apple iPhone 4 had a problem with drop in signal if held in a certain way.

But the company has said an update to its operating system - iOS 4.0.1 - "improves the formula to determine how many bars of signal strength to display".

The release came as Apple said it would be holding a press conference at its Cupertino headquarters on Friday to discuss the iPhone 4.

The company started contacting US-based technology journalists on Wednesday, inviting them to the hastily convened press conference.

Apple has given no further clues about the nature of the event, but has confirmed that the key topic for discussion will be iPhone 4.