Citing figures from Juniper Research, the eBay unit says mobile payments are poised to reach $200 billion by 2012. Merchants want to take advantage of this but "getting in the game has proved difficult".

The Mobile Express Checkout lets customers make purchases with just a few clicks, without having to re-enter billing, shipping and payment type data.

PayPal says its one-stop payment service gives merchants access to millions of mobile shoppers and is easy and quick to integrate with strong fraud prevention.

It is initially only being rolled out on iPhone and Android 2.0 (and higher) devices by a limited number of retailers, including Nike and Buy.com, ahead of a full deployment later in the summer.

Jeff Wisot, VP, marketing, Buy.com, says: "We're seeing a huge opportunity in mobile commerce right now. PayPal's Mobile Express Checkout will allow us to enhance the convenient shopping experience for on-the-go customers."

PayPal has been investing heavily in m-payments with impressive results, seeing mobile transactions increase nearly six-fold in a year, from $25 million in 2008 to $141 million in 2009.

A new version of its iPhone app launched earlier this year was downloaded over a million times in its first three weeks while Mobile Payments Librarys - which let developers add checkout functionality for physical goods and services to their apps - have also been launched.