But since last summer's Google Checkout launch, rival system PayPal, which is owned by internet auction site eBay, has continued to increase its membership.
In response the search engine giant has sought to attract retailers to sign up to its Checkout service by offering them incentives if they advertise via its site.
Salar Kamangar, vice president of product management at Google, said: "By integrating the checkout process with search and advertising, we're helping our users complete the cycle of searching, finding and buying."
It is thought that a European-wide rollout of Google Checkout will take place later this year.
In 2006 Google posted full-year earnings of almost $3 billion (£1.5 billion).
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