Anheuser-Busch is in preliminary talks with Mexico's Grupo Modelo about a potential deal that could thwart InBev's unsolicited takeover bid for the brewer.
Anheuser approached Carlos Fernandez, chief executive of Modelo and an Anheuser director, about a deal in recent weeks, after news surfaced that InBev was weighing a bid for Anheuser, the Wall Street Journal reported.
Anheuser already owns a roughly 50% non-controlling stake in Modelo and a combination of the two could make the company too big to be absorbed by InBev.
This comes a day after Belgium's InBev, which makes Stella Artois and Beck's beers, announced a $46.3 billion bid for top U.S. brewer Anheuser-Busch, which makes Budweiser.
Meanwhile, all three major rating agencies on Thursday said they may cut their ratings on Anheuser-Busch
"We believe that if the transaction proceeds, and includes a significant amount of debt financing as indicated by InBev, credit measures will weaken well below Anheuser-Busch's current levels," Standard & Poor's said in a statement.
S&P and Fitch Ratings both rate Anheuser "A," and Moody's Investors Service rates the company "A2," all the sixth-highest investment grade.
"While it is unclear if the bid will go through and it could take some time for more certainty to develop, there are several ways in which the bid could pressure ratings," Moody's said in a statement.
It would be negative for Anheuser's ratings if InBev incurs substantial leverage to make the deal happen and is itself less creditworthy than Anheuser, or if it levers up the U.S. company to use the U.S. cash flows to repay the debt, Moody's said.
InBev Would Have to Raise: Analysts
The InBev offer amounts to $65 a share, a more than 11 percent premium of Anheuser's closing price on Tuesday and 24 percent above the price before reports of a possible bid emerged last month.
Analysts told CNBC earlier that InBev would probably need to ratchet up that bid.
"We respect the Anheuser-Busch board a lot ... we admire them a lot and we think that the business rationale is very strong," InBev Chief Executive Carlos Brito said in a video statement on InBev's website.
Brito said a combined Anheuser-InBev would retain top managers and board members from both sides, suggesting InBev was looking for a friendly merger of equals rather than a hostile takeover. "The combined company would be run by the combined entities," he said, without divulging whether he would seek to head the new beer behemoth himself.
Despite the fact that the deal would be the biggest ever alcoholic drinks takeover, InBev should have no problem raising the money, Trevor Stirling, senior research analyst for European beverages at Sanford Bernstein told "Squawk Box Europe." It's also unlikely another brewer or beverage company would come in and try and hijack the deal, he added.
"I don't think they would have gone public with this if they didn't have reasonable comfort from the banks," Stirling said.
"This company has a very good track record in paying off its debts and very strong cash flows and clearly the bankers feel comfortable about what's being proposed," he added.
Anheuser dominates the U.S. beer market with a 48.5 percent share.
Anheuser, the maker of Budweiser and Michelob, counts Warren Buffett's Berkshire Hathaway as its second-largest shareholder with a 5 percent stake.
Belgium-based InBev, formed by the 2004 merger of Belgium's Interbrew with Brazil's AmBev, said it would "like to engage in a dialogue with the goal of consummating a friendly combination."
St. Louis-based Anheuser said its board of directors "will evaluate the proposal carefully and in the context of all relevant factors, including Anheuser-Busch's long-term strategic plan", and make a determination regarding the proposal "in due course."
Anheuser and InBev already have deals whereby Anheuser distributes InBev beers including Bass Pale Ale, Hoegaarden and Leffe in the United States and InBev distributes Anheuser beers in Canada.
There have already been arguments against an InBev deal, saying it would cost American jobs and end American ownership of an iconic brand.
Missouri Gov. Matt Blunt said Wednesday he opposes the InBev deal, and the Republican directed the Missouri Department of Economic Development to see if there was a way to stop it, the Associated Press reported.
But shareholders are likely to look at the advantages of a merger, such as the possibility of U.S. products to be sold in Europe and Asia and the benefits of cost cuts, Gower said. And, they'll now have to consider a potential tie-up with Modelo.
The beer industry is undergoing a wave of consolidation, with Scottish & Newcastle agreeing to be broken up by Carlsberg and Heineken and SABMiller and Molson Coors Brewing agreeing to merge their U.S. operations.
If InBev buys Anheuser for $46 billion it will be the largest merger or acquisition this year, excluding spin-offs, and would be the third-largest ever foreign takeover of a U.S. company.
-- The Associated Press, Reuters and the Wall Street Journal contributed to this report