InBev, the Belgian maker of Stella Artois and Beck's beers, is expected to raise its offer for Anheuser-Busch to as much as $50 billion (£25.6 billion) in a bid to persuade the American brewer's board to recommend a deal.

Analysts believe that the brewer of Budweiser will reject last night's opening gambit of $65 a share, worth $47.5 billion, but will be forced by institutional shareholders to enter talks if the bid is raised to nearer $70. Including debt, the current offer is worth almost $57 billion.

The founding Busch family speaks for less than 4 per cent of the shares but controls the boardroom and August Busch IV, the chief executive, said last year that the 156-year-old company would not be taken over "on my watch".

But industry observers believe that, with some members of the Busch family having expressed a willingness to entertain an approach, the board will find it hard to rebuff a raised offer from InBev.

Legal experts gave warning that the board could leave itself open to litigation from shareholders such as Warren Buffett, who holds about 5 per cent, if it rejects a reasonable offer.

Shareholder James Mayfield has already threatened to bring a class-action suit and sought a court order preventing the board from "adopting, implementing or instituting any defensive measures" that would make the company more costly or difficult to acquire.

There have been suggestions that Anheuser might attempt to thwart InBev by seeking to buy out the other 50 per cent of Modelo, the Mexican brewer behind Corona, that it does not already own.

Carlos Brito, chief executive of InBev, declared this morning that the “time is right” for its $47.5 billion bid for its American rival.

He said the business rationale behind the bid was strong and a “natural step”, as he reassured workers that there would be no closures of US breweries as a result of the takeover.

Mr Brito added that the opportunity to join forces with the Budweiser brewer was too good to miss, and that the tie-up would enable both parties to achieve more than they could separately.

In an attempt to win over the Busch family, InBev has said it would turn Budweiser into its flagship beer brand and the name of the combined company would be changed "to evoke Anheuser-Busch's heritage".

The new company would be run by a combined InBev-Anheuser entity, with board members from both companies, and St Louis would remain the home of its North American division.

InBev’s shares rose 3.24 per cent, or €1.35, to €48.82, today in Brussels in reaction to the deal, announced after US markets closed yesterday.

While the offer, which values the shares of the world’s third biggest brewer at $65 each, technically represents an 11 per cent premium to yesterday's closing share price, well-sourced rumours of such an offer have lifted the stock from as low as $45.68 in March.

In a statement last night, InBev claimed that the approach valued the brewer at a 35 per cent premium, before any rumours of a deal had surfaced.

Such a takeover, if successful, would raise hopes in US and European capital markets that the worst of the credit crisis may be over, with such a substantial deal coming soon after Verizon’s offer last week to acquire Alltel, America’s fifth biggest mobile phone company, for $27 billion.

The large transactions may be a sign that banks are again prepared to lend capital after the severe credit downturn.

Anheuser said that it would "evaluate the proposal carefully and in the context of all relevant factors, including Anheuser-Busch's long-term strategic plan". It said the board was mindful of its fiduciary duty to all stockholders.

In an internal memo to employees, Mr Busch suggested that it could be some time before the company reached a decision. "The review process often takes several months or, in some cases, longer due to necessary corporate governance, legal and regulatory steps that must be followed.

"Public announcements, disclosure to shareholders and regulatory reviews must all take place before any change of control can occur. There is nothing you should be doing, other than continuing to focus on business as usual."

InBev, which owns Stella Artois beer and Beck’s, wants to buy Anheuser-Busch because it has little business in the US, the world’s biggest beer market in terms of profits. It would also help it to gain a foothold in China, the largest beer market by volume.

Before the announcement of InBev’s offer, shares in Anheuser-Busch surged by $2 in afternoon trading on Wall Street as fresh rumours began to circulate, closing up 2 per cent to $58.35. Such a move before a offer announcement is likely to attract the attention of the Securities and Exchange Commission, the US financial regulator.

In premarket trading this morning, Anheuser-Busch shares were up $4.65, or 8 per cent, at $63, just below the offer level.

A merger between the two groups is unlikely to face much opposition from the competition authorities because the companies share little geographical overlap.

While Anheuser-Busch has interests in China, Britain and Mexico, where it owns 50 per cent of Modelo, the Bud Light owner derives 86 per cent of its revenues from America, where it has 48.5 per cent of the beer market.

InBev is the biggest brewer in the UK and continental Europe and has a strong South American presence, after its merger with AmBev, of Brazil. The two companies work together in America under an agreement whereby the US brewer distributes some of its Belgian rival’s brands, including Beck’s, Bass and Stella Artois.

InBev, which is being advised by Lazard and JP Morgan, said it had lined up $40 billion of debt funding from Banco Santander, BNP Paribas, Deustche Bank, Fortis, ING, JP Morgan and Royal Bank of Scotland.

It said it would fund the balance of $6 billion from a combination of asset disposals and equity financing.

Anheuser-Busch traces its origins back to the Bavarian brewery, which was established in 1852. Eberhard Anheuser acquired the brewery in 1860 and renamed it E. Anheuser & Co. In 1864, his son-in-law, Adolphus Busch, joined the company.

InBev said the combined company would be one of the top five consumer products companies in the world, with global beer volumes of 460 million hectolitres, net sales of $36.4 billion and underlying earnings of $10.7 billion.

It said that because of the complementary nature of the two businesses, synergies would come largely from sharing of best practice, economies of sacle and rationalisation of overlapping corporate functions.

Analysts estimate the synergies at between $900 million and $1.3 billion.

Trevor Stirling, drinks analyst at Sanford C Bernstein, played down the likelihood of a counterbid. He said SABMiller would be prevented from bidding by the impending deal with Molson Coors to merge their US operations, while Diageo was "highly unlikely to pay a signficant premium for a mature US asset",

He added: "We believe Anheuser-Busch has few options other than to negotiate the sale as we think it is unlikely that other global brewers will act as white knight."

One of Anheuser's few escape avenues could be political. The Republican Governor of Missouri, Matt Blunt, directed the Missouri Department of Economic Development to see if there was a way to stop it.

“I am strongly opposed to the sale of Anheuser-Busch, and the offer to purchase the company is deeply troubling to me,” he said.

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