Google is doubling the size of its Web mail inbox to 2GB, and has threatened to continue to increase it further in the coming weeks.

The announcement is designed to undermine rivals who were left standing this time last year when Google announced its Gmail service would offer a previously unheard-of 1GB for free. The accepted business model at the time was to offer a small inbox space and charges users for greater capacity.

However, there are no plans at the moment to increase Gmail's current 10MB attachment size limit, or to add features that could effectively turn Gmail into an external hard drive.

"Since we introduced Gmail, people have had a lot of place to store e-mail, but some of our heavier users have been approaching their limits and have been wondering what's going to happen," said Georges Harik, Gmail's product management director. "So, starting Friday, we're going to give people more and more space continuously and indefinitely on Gmail, as we're able to technologically."

The percentage of Gmail users approaching their in-box storage limit isn't large, but "they are a number that we care about," Harik said. "We want our users to understand that we have a plan and that we're anticipating their needs, and that there isn't something strange that's going to happen with Gmail down the line."

Since Gmail's arrival, most major Web mail providers have reacted by signficantly increasing the free storage offered. Microsoft and Yahoo now both offer 250MB, and Yahoo plans to begin offering 1GB next month. Both Yahoo and Microsoft offer 2GB with their charged-for Web mail.

Gmail currently remains in beta. And while it is free to use, it features text ads that are served up to users with each message they open, based on each message's text.