Millwall's Under 21s were beaten 4-2 by Cardiff City in the Professional Development League on Tuesday afternoon.
Frankie Hvid's header levelled things up at 1-1 in South Wales after The Bluebirds took the lead, but two further strikes from the home side put the game beyond Kevin Nugent's men after the break.
Seb Drozd reduced the arrears late on against a strong Cardiff side, containing first-team players, but a further effort restored the hosts' two-goal advantage as they saw the game out.
The loss is just the third of 2022/23 for The Lions, with Bristol City a point in front of Millwall - and with a game in hand - at the summit of the PDL at the time of writing.
Millwall Under 21s' next fixture sees Reading visit on Monday 27th February.
Millwall: Wright, Walker, Adom-Malaki, O. Evans, Hvid, Okoli, Allen, Cotton (Heywood 78'), Hearn, Abdulmalik (Coleman 63'), Drozd.
Subs not used: G. Evans, Stephenson, Massey.
Scorer: Hvid 43', Drozd 82'