Microsoft is threatening to launch a boardroom coup at Yahoo! within six weeks if the internet search engine fails to accept its $45 billion (£23 billion) hostile takeover proposal or start serious merger talks, The Times has learnt.

The plan to oust Yahoo! executives came as it emerged that the search engine was considering the feasibility of a tie-up with Google, its bigger rival, to fight off the approach from Microsoft.

Although a source close to Google said that it was not considering a counterbid for Yahoo!, it is believed that the Yahoo! management is mulling over some kind of co-operation with the world’s largest internet company.

Over the weekend, Eric Schmidt, the Google chief executive, apparently rang Jerry Yang, his counterpart at Yahoo!, to discuss the possibility of a partnership through which the two might fend off Microsoft.

It is understood that Google executives have also been calling colleagues at media companies such as Time Warner to discuss whether they are planning to table a rival bid to Microsoft's, though at this stage none seems likely.

While any tie-up between the two would trigger a cartel investigation — a combination of Yahoo! and Google would have more than 80 per cent of the UK’s internet searches alone — Yahoo! is thought to be considering options such as turning to Google as its search provider.

Goldman Sachs, the main adviser to Yahoo!, declined to comment, and Lehman Brothers, its other adviser, was not immediately available.

At the weekend, Google accused Microsoft of seeking to extend its computer software monopoly across the internet and called on policymakers around the world to challenge such a takeover.

David Drummond, Google’s chief legal officer, said in a blog that a combination of Microsoft and Yahoo! could undermine the open competition that has fuelled more than a decade of innovation on the web.

He said: “Could a combination of the two take advantage of a PC software monopoly to unfairly limit the ability of consumers to freely access competitors’ e-mail, IM and web-based services?”

It is understood that Microsoft executives have already agreed a “sledgehammer” tactic of ousting most of the Yahoo! board to force through a deal quickly if Mr Yang, a Yahoo! co-founder, fails to recommend a takeover or begin earnest negotiations about a tie-up.

Microsoft is prepared to use its right as a Yahoo! shareholder to nominate its own executives to the board and then force the company to accept the Microsoft takeover proposals.

Under Yahoo! bylaws, any shareholder has the right to nominate executives to be voted on by all shareholders. The deadline to nominate directors this year is March 13.

Although it is understood that Microsoft would much prefer a takeover to be amicable and not resort to ousting Yahoo! directors, the software company has become increasingly frustrated that the lack of a tie-up between the two groups has helped Google, the world’s biggest internet company, to grow even stronger.

So anxious are Microsoft executives about Google’s dominance that they are prepared to remove the Yahoo! board.

Microsoft is believed to be confident that its nominations would be backed by other shareholders because of the size of its bid premium and because Yahoo! has suffered eight consecutive quarters of falling profits and shrinking market share.

On Friday Microsoft made a $45 billion cash and shares takeover approach for Yahoo!.

The fully funded, hostile approach values Yahoo! shares at $31, a 62 per cent premium to the closing price on Thursday evening.

Microsoft has been seeking a tie-up with Yahoo! for more than a year. Last February talks broke down after the Yahoo! board declared that the company was not for sale.

The internet group did promise, however, to devise plans so that the two companies could co-operate and compete more effectively against Google. Microsoft believes that Yahoo! has failed to keep its side of the bargain.

Some analysts say that although a bid valuing Yahoo! at a 62 per cent premium is hard to reject, Yahoo! may consider a tie-up with another media group, such as News Corporation, parent company of The Times and Times Online, or Disney, or a telecoms group, such as AT&T.

Times Recommends

• US stocks fall below 9,000 mark
• House price fall smallest in seven months
• Wary investors opt to keep cash in safes