Most BlackBerry users are now up and running again. Research in Motion (RIM), which makes the device, has around 45% of the global market for 'smart' phones.

The company now has eight million users worldwide. Technology commentators told the BBC that the network may have reached its capacity, causing the breakdown in service. If this is the case, there may be future implications for UK-based users of the devices.

RIM said it was investigating what had caused the problem.