Google is scrambling to identify what caused its global mail service to fail at around 1.30am Pacific Time.
"A number of users are having difficulty accessing Gmail. We are working to resolve the problem," a spokesperson from the company's UK press office told Computer Weekly.
Google remains unclear as to whether the problem is hardware or software related, but said that a statement would be made once more information comes to hand.
Google mail has several million users worldwide, with the service generating significant advertising revenue each year.
In January, Google reported fourth quarter revenues of US$5.7bn.
>> Read more: Google mail Gmail is down - Is this Gfail?