Cardiff City fumed at the decision to disallow an acrobatic strike from Sol Bamba as the Bluebirds missed an opportunity to put pressure on their promotion rivals.
Victory in south London would have lifted Neil Warnock's side level with second-placed Derby County and Bamba's 84th minute volley looked to have done just that.
However, referee Keith Stroud had already brought play back for a foul on Joe Bennett, denying the visitors a much-needed three points.
The draw leaves Cardiff two behind Derby and one adrift of third-placed Aston Villa, although the Welsh club know they will still have a game in hand after this weekend's fixtures.
Things had looked much more positive when Junior Hoilett opened the scoring after tfour minutes, the forward robbing Mahlon Romeo before lashing beyond Jordan Archer.
However, Cardiff failed to make their early control pay and Millwall restored parity as Lee Gregory fired a first-time effort beyond Neil Etheridge from Jed Wallace's low cross.
Ben Marshall, Wallace and Gregory all had good openings early in the second half as the hosts sensed victory, but Cardiff were the ones smarting at only coming away with a point.
With time running out, Bamba thought he had won it, only for the referee to award a free-kick, much to the ire of Warnock.
"It is appalling at this level, I feel really let down," the Cardiff boss told Sky Sports after the match. "The official has cost us, [it is an] absolute disgrace."