A 19th-minute goal from in-form striker Jermaine Beckford gave the League One (third division) leaders victory as United lost at this stage of the competition for the first time since third division Bournemouth beat them in 1984.
Beckford struck his fifth goal in three games and 20th of the season when a 50-metre ball from Johnny Howson went over the champions' defence, allowing him to control it and angle a shot wide of goalkeeper Tomas Kuszczak. It was the first time Alex Ferguson had suffered defeat in the third round, the stage at which the top clubs enter the FA Cup, since he became United manager in 1986.
United's first lost to lower league opposition since 1984 also gave Leeds a memorable victory at a ground where they regularly used to duel with United for national supremacy before their slide down the leagues.
On Saturday, Roberto Mancini, the King of the Italian Cup, took his first step towards Wembley when his Manchester City side earned a 1-0 win at Championship side Middlesbrough. There were no real upsets on Saturday, although Liverpool had to come from behind to draw 1-1 at Championship side Reading and Portsmouth, the Premier League's bottom team, were held 1-1 at home by Coventry City.
Last season's runners-up Everton beat Carlisle United 3-1 while Tottenham Hotspur eased past Peterborough 4-0.
Championship side Preston North End, who sacked manager Alan Irvine last week, thrashed Colchester United 7-0 with a hat-trick by Jon Parkin while Bolton Wanderers, also managerless after unloading Gary Megson, beat Lincoln City 4-0.
In two all-Premier League ties Aston Villa beat 10-man Blackburn Rovers 3-1 and Wigan Athletic defeated Hull City 4-1.
There was a hat-trick of defeats for minor league sides as Barrow went down 3-0 at Sunderland, Luton Town lost 1-0 at Southampton and York City, having briefly led at Premier League Stoke City after arriving late due to traffic, lost 3-1.
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