Millwall's Under 23s were defeated 3-1 by Colchester United in the Professional Development League on Monday afternoon.
The Lions' took the lead through an own-goal, but The U's raced to victory from there, courtesy of strikes from Dean Ager, Matt Weaire and Louis Dunne.
Millwall were in front just 12 minutes in, after good work between Tyler Burey and George Alexander set up Reuben Duncan inside the area - the ball was then turned home by a Colchester defender, with Duncan claiming the credit.
Shortly before half-time, however, the hosts were back on level terms after Ager's left-footed effort beat Sandford from around 20 yards out.
Just past the hour-mark, The U's were in front after Weaire converted from close range following the ball striking the post, before Dunne's late penalty sealed all three points for Colchester.
Next up for Millwall's Under 23s is a Professional Development League meeting with Coventry City at The Den on Thursday 10th October - ticket details will be confirmed in due course.
Millwall: Sandford, Moss, Tiensia, Mitchell, Ransom, Topalloj, Burey, Skeffington (Davis 65'), Alexander, Duncan, Trialist. Subs not used: Strachan, Fanshawe, Muller, Munting.