Google’s advertising deal with eBay, which will see both sides trial "click-to-call" ads for the first time, is likely to be the largest of the raft of online agreements signed in recent weeks, according to industry analysts.

The two sides refused to disclose financial details of the tie-up, through which Google will also provide text-based adverts across eBay's websites outside the United States.

But analysts suggested that the sums involved will beat the $900 million payment guarantee made by Google to News Corporation, owner of The Times, when the pair agreed a deal making the search engine the advertising supplier to a group of sites including MySpace.com earlier this month.

"The sum was undisclosed, but must be gargantuan," the Techcrunch blog said.

In terms of user numbers covered, the two deals are on a par. EBay currently has around 203 million users worldwide – around half of which are estimated to be outside the US. Growth rates outside America are believed to be twice those in its domestic market.

MySpace has more than 100 million users worldwide and is also growing rapidly.

However, analysts pointed out that most eBay users visit the site intent on spending money, making them a more valuable commodity for advertisers. MySpace has grown as a forum where young people meet up online - making it a strong platform for launching new pop talent but less of a marketplace.

It has also been noted that Google has shown itself ready to offer favourable terms in order to snag prime online assets for its own advertising services.

Under the terms of the deal made public, Google and eBay will also co-operate on developing a "click-to-call" advertising model - which would put buyers directly in touch with sellers via a voice call when they click on an advert.

Pilot schemes have indicated that some advertisers are willing to around ten times as much as for a click that activates a phone call as for a traditional click on an ad that leads through to another web site.

"The vast majority of businesses still serve customers via the phone," eBay chief executive Meg Whitman said. "This is the online equivalent. Google and eBay are uniquely positioned to make this market a reality."

Additionally, Google's desktop toolbar will be bundled with Skype, the internet telephony software that eBay bought last year for up to $4.1 billion. Google also has the option to deliver graphical adverts - on a non-exclusive basis - on eBay's international sites.

Wall Street welcomed the deal, with positive analyst comment helping to send both sets of shares higher at yesterday's close. Ebay gained 49 cents to $25.79. Google jumped $7.69 to $380.95.

Mark Rowen of Prudential Equity Group said: "Google adds a major traffic partner to its international network, while eBay better monetises its European sites."

However, eBay said that the agreement would not make an impact on its bottom line in the next two years, leading to speculation that Google did not offer a revenue guarantee this time and that payments will depend on targets set over at least the next 24 months.

The deal comes as a blow to Yahoo, which earlier this year signed an agreement to provide advertising for eBay’s US site, but which has been hit by delays to its next generation of advertising technology.

Both the MySpace-Google and eBay-Google deals have also eclipsed Microsoft’s tie-up with Facebook, a smaller rival to MySpace, that was announced last week.

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