A woman carries beer produced by brewing company SAB Miller past kegs at a bar in Cape Town Thomson Reuters By Tiisetso Motsoeneng and Martinne Geller

JOHANNESBURG/LONDON (Reuters) - Three of SABMiller's top 10 shareholders have spoken out in support of the brewer's board spurning bigger rival Anheuser-Busch InBev's $100 billion takeover offer, piling pressure on the maker of Budweiser to raise its bid.

Investors on Monday were expecting an improvement to AB InBev's last offer, which at 42.15 pounds per share in cash for most shareholders was rejected by SABMiller as "very substantially" undervaluing the company.

A higher offer in the range of 43 to 44 pounds a share is a possibility, a source familiar with the matter said on Sunday.

AB InBev has until 1600 GMT on Wednesday to launch a formal bid for the maker of Peroni and Grolsch, in what would be the biggest UK company takeover ever. It has already made three informal offers, which were rejected by SAB's board excluding the three directors nominated by cigarette maker Altria Group , SAB's largest shareholder with a 27 percent stake.

South Africa's Public Investment Corporation (PIC), Poland's Kulczyk Holding and Scotland's Aberdeen Asset Investments - respectively SABMiller's fourth, fifth and seventh-largest shareholders - have all publicly sided with SABMiller. Altogether their holdings account for 8.2 percent of the company.

"We have confidence in the board and we will rely on their judgment. They have said the price is too low and we agree with them," Dan Matjila, chief executive of PIC, told Reuters.

On Friday Aberdeen called the offer "welcome" but said that "AB InBev need to rethink their numbers".

Still unclear is the ultimate position of Colombia's Santo Domingo family, which owns 14 percent of the company following the sale of their Grupo Bavaria business to SABMiller in 2005.

Their two board representatives who voted against last week's offer are Alejandro Santo Domingo and Carlos Alejandro Perez Davila, cousins who also run New York-based Quadrant Capital Advisors.

Yet Alejandro Santo Domingo, the Harvard-educated fixture of New York high society, is according to media reports well-acquainted with AB InBev's controlling shareholders, including Brazilian billionaire Jorge Paulo Lemann.

"We think the Colombians are very close to Lemann and that Lemann knows exactly what price the Colombians want," said an SAB shareholder with a less than 1 percent stake. "I can't understand their tactics though."

AB InBev Chief Executive Carlos Brito said last week that his company's large shareholders had approached Altria and the Santo Domingos and understood them to be "at least receptive to an approach". He also said the structure of his alternative offer to buy the 41 percent of the company they own for a mix of shares and cash, was created "with and for" Altria and the Santo Domingos.

Santo Domingo could not immediately be reached by Reuters for comment. He told the Financial Times last week that "at the moment we have no comment".

The smaller SAB shareholder, who was not authorized to speak to the press, said he could see merits in a merger and hoped discussions would continue to find a price that was acceptable to Santo Domingo.

"It makes so much sense to put these two businesses together, but they clearly have to negotiate and get the best price," the shareholder said. And at the moment, they need to get the Colombians on board."

($1 = 13.3080 rand)

(Editing by James Macharia and Greg Mahlich)