Gmail, the free email service run by Google, has officially moved beyond beta stage and is open to anyone in the US.

The service had been available only to those invited to join, but now the service is open to any US resident with a mobile phone. New users can sign up for the service and get a code sent to their mobile phone that allows them to open an account.

"Why use mobile phones?" said Google in its corporate blog.

"It's a way to help us verify that an account is being created by a real person, and that one person isn't creating thousands of accounts. We want to keep our system as spam-free as possible, and making sure accounts are used by real people is one way to do that."

The Gmail service gives users 2Gb of free email storage and uses powerful spam filters to keep inboxes from being clogged. In tests at VNUnet.com no spam has been received in over six months, despite the address being posted on public forums.

The system also allows up to 10Mb of data to be sent in a single message, leading to some users using it as a music sharing service.

When the service launched as a beta last year it caused Yahoo and Hotmail, the two leading free web email services, to increase the levels of storage they offered free to users.