SABMiller has confirmed that AB InBev had informed the board of its intention to acquire SABMiller, but said it had not yet received an offer.
The combined value of the two firms is likely to be at least $230bn (£150bn) based on Tuesday's share price.
AB InBev has until 5pm on 14 October, according to London rules, to announce its intention to make an offer.
AB InBev's brands include Budweiser, Stella Artois and Corona, while SABMiller owns Peroni and Grolsch.
AB InBev and SABMiller are the world’s two largest brewers and, if the deal is successful, the merged company would produce one third of the world's beer.