By FIDE rules, I think you would have to play it out. The rules specifically account for the fact that you could theoretically allow the promotion therefore losing on time would lose the game.
If it was under US Chess rules, it would be a draw on the pawn capture I believe. I would have to look it the specific regulation but they require a forced win in a case like that.
Some time ago I was in a tournament where in a penultimate round I reached an endgame of bishops and pawns. At the time I (playing black) was better but lost my bishop after a classic pawn-bait trap (taking on a2, then white plays b3, and win the bishop). Luckily, my opponent was fairly innocent enough to let me reach this position. (I can't remember the position but it looks like this)
I just checked his king with his last pawn, and he grabbed it. I immediately called the arbiter, claiming it is a draw of wrong rook pawn ending, and the arbiter agreed. At the time he agreed, I have only 50 seconds left on the clock, while my opponent has more than 1 minute left.
After the game, I realized that despite the well-known 'book draw', I cannot force a 50-move rule or the threefold repetition with only 50 seconds, because if he moves his pawn every 49 moves, then I must play more than 150 moves, meaning that I must do 3 moves of my king and 3 times pressing the clock in one second! Worse, the rules stated that we cannot use both hands in a game (even to press the clock) and the clock is located on my right side of the board (while my king is on my left). Therefore, I conclude that it is practically impossible to force a draw provided that the arbiter was strict enough.
Indeed, I still fell guilty about this game and I think I don't deserve this kind of draw. What do you guys think? Is it a draw?