Google and some of the world's largest social networks have launched their most aggressive attack yet on Facebook, their young rival.

An alliance of companies led by the search engine is planning to introduce a set of common standards that will let software developers write programs for Google's social network, Orkut, and others such as LinkedIn and Friendster.

The strategy draws inspiration from a feature introduced by Facebook earlier this year, which enables outside developers to write applications for the site, which can then be downloaded and shared by Facebook users.

A cross-site, open platform will, Google hopes, be more attractive to developers than Facebook, and siphon off some of the momentum from the rapidly growing site, for which more than 5,000 applications have been written since it opened its platform in May.

A source close to the deal, which is expected to be announced tomorrow, told The New York Times: "It is going to forestall Facebook’s ability to get everyone writing just for Facebook. This is an open alternative to that."

The initiative, called OpenSocial, encompasses not just other social networking sites - like hi5 and Plaxo - but also providers who have traditionally focused on business software, such as Salesforce.com and Oracle.

The breadth of the group shows the extent to which networking sites and other software companies now see the value of open platforms, which allow third parties to design features which can be installed by - and shared between - users.

Apple has said that it will allow developers to write widgets, as the applications are known, for its iPhone, and similar announcements have been made by MySpace and Meebo, another social network, as well as Nokia.

Joe Kraus, director of product management at Google, said that negotiations about the alliance preceded Microsoft's investment in Facebook, adding: "Obviously we'd love for them [Microsoft] to be a part of it." Facebook would not comment on the story.

Companies increasingly see widgets - which users often recommend to one another - as a more succesful way of targeting customers than traditional advertising, but their business potential remains largely unrealised, analysts said.

"So far most of these applications - like the ones on Facebook - have been made for fun, but we're going to see a growth in them across all sorts of environments," Rebecca Jennings, an analyst with Forrester said.

Google will also be hoping the deal can generate new interest in its social network, Orkut, which is popular in Brazil but has struggled to gain a foothold in the US.

The company's chief executive, Eric Schmidt, has said of the growth of social networks: "If you are of a certain age, you sort of dismiss this as a thing for college kids or teenagers. But it is very real."

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