Aug 12 (Reuters) - Transat A.T. Inc TRZ.TO said on Monday that a Canadian financial markets administrative tribunal had blocked Groupe Mach's offer to buy 19.5% class B voting shares of the Canadian tour operator. this month, Montreal real estate developer Mach had offered to buy at least 6.9 million class B voting shares of Transat, to block a rival takeover offer from Air Canada AC.TO for Transat. Canada AC.TO on Sunday raised its offer to buy Transat by $5 to C$18 per share, topping Mach's takeover bid of C$14 per share.