A small green shopping-trolley icon now appearing next to selected Google ads could be a big boost for the internet search leader's relationship with advertisers, who provide 99 per cent of its revenues.

Google Checkout, which launched last week, is a fast-track system for consumers to complete transactions on the websites of its customers, according to the Silicon Valley company.

But it also represents a move into online payment services and gives Google the vital end-to-end experience of knowing what happens when users click on its ads.

That in turn could lead to a changing business model. If Google knows how many of these click-throughs lead to purchases, it can charge higher rates based on whether the sales are completed.

Charlene Li, internet analyst with Forrester Research, says this "cost per action" model is one towards which both Google and its customers are gravitating.

"I do believe that they will be moving towards a cost-per-action advertising model; that's what retailers really want. If I sell $100 worth of goods, what's that worth based on how well I am converting [the click-throughs to my website into sales]?"

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