Shaun Batt flew off the substitutes' bench to score the only goal and deal Watford's promotion bid another blow.
Millwall are sweating on the fitness of Liam Trotter ahead of the FA Cup semi-final against Wigan.
Birmingham and Millwall shared the spoils in a 1-1 draw dominated by controversy at St Andrew's.
Latest team news ahead of the weekend fixtures with Cardiff dogged by defensive problems before they visit Sheffield Wednesday.
FA Cup semi-finalists Millwall will be without Mark Beevers through suspension when they make the short trip across south London to face Charlton.
Millwall manager Kenny Jackett has an unchanged squad for the FA Cup quarter-final replay at Blackburn.
Millwall and Blackburn will have to do it all again to settle their FA Cup quarter-final tie after a dour 0-0 draw at The Den.
Stephen Warnock's blushes second-half penalty helped Leeds keep their play-off hopes alive with a 1-0 victory over Millwall.
Millwall boss Kenny Jackett was relieved to end a run of five successive league defeats after they beat Middlesbrough 2-1.
Middlesbrough boss Tony Mowbray pointed the finger at his defence after a 2-1 home defeat by Millwall saw his side slip out of the play-off places.
Middlesbrough's play-off hopes were dealt another crushing blow as they were denied a dramatic late equaliser in a 2-1 defeat against Millwall.
Peterborough produced a masterful attacking display to over-run FA Cup quarter-finalists Millwall 5-1 and move off the foot of the npower Championship table.
Benik Afobe and Martyn Woolford return to the Millwall squad for the visit of Peterborough.
Hull got their promotion bid back on track with a first win in four npower Championship games courtesy of a goal inside the opening minute at Millwall.
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