TORONTO—Ontario's financial and political elite are quietly airing concerns over the proposed takeover of Toronto's bourse by its London cousin—signaling the deal could face a long and potentially rocky road to regulatory approval.

The legislature of Ontario on Wednesday approved the creation of a special committee to review the deal, in which shareholders of the London Stock Exchange Group PLC would take a 55% stake of the combined company, while shareholders of Toronto Stock Exchange operator TMX Group Ltd. would get 45%....