Millwall's Under 23s were defeated 2-1 by Sheffield United in the Professional Development League at Calmont Road on Tuesday afternoon.
Zak Brunt gave The Blades the lead from the penalty spot, but Sean O'Brien responded with a penalty-kick of The Lions' own shortly before the hour-mark to level the scores.
Kyron Gordon, however, headed home a free-kick to give Sheffield United the win - and the Professional Development League North championship title.
The Yorkshire outfit began the game on top and almost took the lead as early as the third minute through Will Lankshear, but he glanced a free-kick wide of the post.
Just three minutes later, though, United were in front - from the spot - after Arthur Penney fouled Lankshear. Brunt stepped up to send Joe Wright the wrong way and open the scoring.
Angelo Cappello and Lankshear were proving to be thorns in The Lions' sides as the first-half wore on, and Lankshear had the ball in the net midway through the half, but was deemed to be in an offside position.
As the second half of the first period came into view, though, Millwall began to get on top of the game and advance towards the Sheffield United goal - but the next chance went the way of The Blades as Ryan Viggars headed over.
Kevin Nugent made a double change at half-time, and both substitutes - Tyrese Briscoe and Besart Topalloj - were straight into the attack for The Lions. On 59 minutes, the former was hauled down in the box and the referee gave his second penalty of the game. O'Brien stepped up to send the goalkeeper the wrong way and level the scores.
Minutes later, O'Brien could have had his and Millwall's second, but his well-hit free-kick struck the post and bounced behind.
Millwall were now on top and looking the more likely to score, but with 67 minutes on the clock, The Blades were to get themselves back in front as Gordon nodded home a free-kick from the left-hand side.
Viggars would have put United 3-1 up if not for some brave goalkeeping by Wright, and despite two late chances for Jayden Davis and Reuben Duncan, The Lions suffered defeat as The Blades claimed the League title.
Next up for Millwall's Under 23s is their final match of the season at Hull City next Monday night.
Millwall: Wright, Walker, Moss, Penney, Ransom, Strachan, O’Brien, Duncan, Abdulmalik (Briscoe 46'), Boateng (Topalloj 46'), Davis. Subs not used: Gillmore, Miller.
Scorer: O'Brien pen 59'