Terms of the proposed union are still being worked out, these people said, and remain murky. News Corporation would probably contribute its Fox Interactive Media unit, which includes MySpace, and possibly cash to a partnership with Microsoft as part of an acquisition of Yahoo, they said.

The talks represent a change of sides for Murdoch. Just days after Microsoft made its bid, he flew to the West Coast and had dinner with Jerry Yang, Yahoo’s chief executive, offering his assistance in fending off Microsoft.

A deal between them would have involved joining forces with Google to use its lucrative advertising system on the companies’ sites. Antitrust concerns have, however, all but scuttled those talks, at least in that form.

On Wednesday, Yahoo suggested that it might be willing to cede part of its core business to Google, an archrival, to remain independent.

Yahoo said it would begin outsourcing a small portion of its search advertising to Google. The limited test is meant to determine whether the company could extract more revenue if Google ran its search advertising system. The test results might also back Yahoo’s contention that Microsoft’s offer undervalues the company, a person briefed on the plan said.

In the two-week test, Yahoo will use Google’s search advertising system to deliver ads that appear alongside Yahoo’s search results. The test will involve searches conducted in the US on Yahoo.com, not on any of the company’s search affiliates, and will be limited to no more than 3 per cent of all search queries, Yahoo said in a statement. Yahoo also said there was no guarantee that the test would lead to a broader deal.

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