Which international tournament chess game has the record of longest sequence of forced moves until checkmate? I want to know which international game has the longest sequence of forced moves until checkmate, even if the opponent resigned the game.

Thanks in advance.

Note : Result should not be drawn [1/2 - 1/2] (i.e we should consider 50 move rule).
Alekhine - Yates
http://www.chessgames.com/perl/chessgame?gid=1012179

19 moves.  Alekhine resigned a couple of moves before mate.