A technical problem has kept an undetermined number of Windows Live Hotmail users locked out of their email accounts for hours on Thursday. The problem, which started at around 3:30 am US Eastern Time, has affected "a small amount of customers," a spokeswoman for Microsoft said. She declined to be more specific about the number of people affected.

"We are continuing to investigate the issue, but can confirm that the majority of customers that were affected are now able to access their Hotmail accounts," she wrote.

The outage apparently was large enough to warrant the posting of a prominent note at the top of the Windows Live Solution Center stating that Hotmail is experiencing login issues. The message remains on the page as of 7 pm, meaning the problem is now entering approximately its 16th hour.

Hotmail has about 355 million active accounts worldwide, the spokeswoman said, citing figures from comScore.

Last week, a network problem at a Microsoft data centre affected access to the company's Business Productivity Online Suite (BPOS) and to other hosted software products for enterprises.