Millwall squandered the opportunity to heap pressure on the Championship top six by slipping to a 2-0 defeat to Neil Warnock's Middlesbrough, who moved out of the bottom three with a crucial and controversial victory in south London.
The Lions sit five points outside the play-off places with four matches remaining and their promotion hopes are fading after a third failure to win at The Den since the restart.
Boro, meanwhile, boosted their survival chances and moved ahead of Hull on goal difference after making in two wins and two defeats under veteran Warnock.
On balance, the visitors deserved the three points after a ponderous display from Millwall in the final third, but the victory was not without controversy after two second-half decisions by referee Tim Robinson proved decisive.
The hosts were incensed by Britt Assombalonga's 68th-minute opener, as neither Robinson nor his linesman signalled for a foul when Jake Cooper appeared to be caught by Ashley Fletcher.
The Lions centre-half went sprawling to the rain-soaked turf but Robinson waved play on and Fletcher charged down the right flank and showed a cool head to square for the Congolese striker to finish from six yards.
It was a textbook sucker punch, coming just as the hosts were beginning to control the contest following a triple substitution made by manager Gary Rowett on the hour.
Millwall could have fewer complaints when Fletcher was hauled down by Cooper with four minutes to go and Robinson pointed to the spot. Cooper was lucky to escape with just a yellow card and Fletcher hammered his penalty down the middle.
A 3pm BST midweek kick-off – agreed by both clubs and the EFL to minimise travel disruption – gave both clubs the opportunity to steal a march on their immediate rivals but Millwall failed to build on the 1-0 win at Charlton last time out, their only victory in five matches since the restart.
They remain 10th in the table on both home and away form, and if they fail to make the top six it will be down to frustrating results at The Den, where they have not been clinical enough.
Boro started the brighter and were convinced they should have had a penalty when Djed Spence appeared to be caught by Murray Wallace as he raced to the byline, only for Robinson to award a goal kick.
Right-back Spence was a thorn in Millwall's side and ten minutes later his low cross only needed a touch. Paddy McNair then stung the palms of Millwall goalkeeper Bartosz Bialkowski from Assombalonga's cute free-kick.
Boro needed Ryan Shotton to make a crucial intervention to deny Ben Thompson – Millwall's liveliest player – in the area but the hosts' only significant chance of the first half was after 13 minutes when Matt Smith headed over a Shaun Williams corner.
Within seconds of the restart, McNair should have given Boro the lead when he blazed over Bialkowski's crossbar after a slick move down the left.
By the hour, Rowett had seen enough and rolled the dice with a triple change, introducing Tom Bradshaw, Jayson Molumby and Connor Mahoney from the bench.
It nearly had the desired effect within seconds as Mahoney's cross found Smith, whose header was superbly saved one-handed by Dejan Stojanovic.
Moments after, Mahoney – who did more in five minutes than many of his teammates in 90 – cut inside from the left and saw a stinging low drive saved by the Macedonian goalkeeper.
Just as it began to look as though Millwall would find a breakthrough, Fletcher broke clear to set up Assombalonga's sweeping close-range finish.
As Millwall pushed for a response, the former Manchester United and West Ham youngster got away from the hapless Cooper again before ending the contest – and perhaps Millwall's play-off hopes – from 12 yards.