Yahoo is to celebrate the 10th anniversary of its online email service by offering unlimited storage to users. 

John Kremer, vice president for Yahoo Mail, said on a company blog that existing mailboxes will be upgraded from May. 

"As much as we would like to just flip a switch and 'unlimit' everyone on the same day, we will be rolling this out over a few months to facilitate a smooth transition. We know there is virtually nothing more precious than your inbox."

The move puts Yahoo Mail ahead of Google's Gmail in terms of storage capacity. Google currently caps users at 2.8GB and is gradually adding storage capacity.

When Yahoo Mail first launched in 1997, mail boxes were limited to 4MB of storage. The internet portal responded to Google's offering of 1GB mail boxes in 2004 by increasing the capacity to 100MB, which was increased to 1GB a year later.

Yahoo is the world's largest provider of web-based email services with a slight lead over Microsoft's Hotmail, according to comScore Media Metrix.