With Tranmere Rovers beating Millwall 1-0 at Prenton Park, Leeds would have guaranteed Championship football next season with victory over Charlton Athletic at The Valley.
The Yorkshire side, however, could not get that vital win as they were defeated 1-0 by the Addicks.
It sets up a tremendous final day of the season in League One next Saturday, with five teams still in contention for automatic promotion.
Leeds remain in pole position on 83 points, if they beat Bristol Rovers at Elland Road they will be promoted.
But they will know they must win, as Millwall and Swindon - who will play each other at the New Den - are both on 82 points.
Should Millwall and Swindon draw, then Charlton could still overtake their three promotion rivals to finish second if they are victorious in their trip to Oldham Athletic.
Huddersfield Town, currently sixth, could even possibly take second should they win at Exeter City. They would need Leeds and Charlton to both lose, and Milwall and Swindon to draw - a scenario which would leave them as one of four teams on 83 points, where goal difference would be the deciding factor.
Norwich City have already been confirmed as League One champions.