LONDON (Reuters) - Investor advisory firm Glass Lewis has recommended SABMiller <SAB.L> shareholders vote in favor of its takeover by Anheuser-Busch InBev <ABI.BR>, adding to the likelihood of the $100 billion-plus beer mega-merger getting done.

One of the biggest deals in corporate history, which will marry brands such as Castle Lager and Corona, has been endorsed by both companies' boards. It will face a shareholder vote on Sept. 28.

Glass Lewis said the cash offer was a fair price at which SABMiller shareholders could cash out and immediately realize a significant premium. It also said the deal represented a historically high valuation multiple.

"Based on these factors, along with the unanimous support of the board, we believe the proposed acquisition is in the best interests of shareholders," the firm said in its report.

(Reporting by Martinne Geller; Editing by Mark Potter)