T-Mobile has said that a software fault on one of its back-end systems was responsible for a loss of mobile phone coverage for millions of its customers.

The company's mobile phone base station started to malfunction yesterday and cut off an estimated five per cent of its 33.5 million US customers from voice calls and messages. However, judging from the outrage on forums, the figure may be much higher.

"On Tuesday, some T-Mobile customers may have experienced service disruptions impacting voice and messaging services. We restored full service to all affected customers later in the day," said the company in a statement.

"After investigating the cause, we have determined that a back-end system software error had generated abnormal congestion on the network. T-Mobile has since implemented additional measures to help prevent this happening in the future. We again apologise to those customers who were affected and may have been inconvenienced."

The outage was doubly embarrassing for T-Mobile after customers using the Sidekick platform lost their data after a mistake by Microsoft, although the information was later restored.

The precise cause of the crash has not been announced, but it may be similar to the 1990 outage that took down the AT&T phone network.

In the AT&T case, a software upgrade malfunctioned, registering phone connection nodes as busy and passing traffic onto other parts of the network. The cascade effect as more and more nodes received the upgrade took out almost the whole network.

However, yesterday's T-Mobile crash had the potential to be much more serio us, as a third of US homes and businesses have eschewed landlines in favour of mobile phones.