SABMiller, the world's second-largest brewer, promptly rejected an improved takeover proposal from Anheuser-Busch InBev, saying its $104 billion valuation was "very substantially" under par.

Refusal of the offer, made public on Wednesday after earlier proposals were refused privately, opens the door to a week of intense wrangling before an Oct. 14 deadline for a formal bid set by Britain's takeover panel.

The deal would be one of the biggest in corporate history, uniting the maker of Budweiser, Corona and Stella Artois beers with that of Peroni, Grolsch and Pilsner Urquell. The combined entity would make nearly one-third of the world's beer.

AB InBev realistically needs access to SAB's private financial data if it is to make an informed formal bid but said that so far the board had not engaged meaningfully.

A deadline extension can be granted if SABMiller asks for it, and AB InBev wants shareholders to lobby the company.

"The deadline is approaching, and we thought we should make it public," AB InBev CEO Carlos Brito said. "Now it's up to the shareholders to have a look at it. If they think this is a good offer, they should act and encourage the board to engage."

Brito said he was committed to a friendly deal but did not rule out going hostile.

AB InBev said it lacked the support of BevCo, controlled by the Santo Domingo family of Colombia. Representatives for BevCo, which owns about 13 percent of SABMiller, could not be reached by Reuters. BevCo has two seats on the SABMiller board.

SABMiller said its board, excluding the three members nominated by Altria, unanimously rejected the proposal.

"It still very substantially undervalues SABMiller, its unique and unmatched footprint, and its stand-alone prospects," the U.K.-based company said in a statement.