Csaba Laszlo’s side crumbled amid the heat after nearest rivals St Mirren had successfully held their own nerve by amassing an 11-point lead at the summit of the Championship, thanks to a shrewd 2-1 victory at Dunfermline the previous night.
Just when it was their turn to show mettle, United instead failed miserably despite the magnitude of what was at stake on the day.
Any notion of them now paving an automatic route back to the Premiership seems wayward to say the least while they stare at the prospect of yet another round of dreaded play-offs having lost out at precisely that stage last season.
By the time Jack Iredale’s 80th minute solo effort had bulged Harry Lewis’ net, there was a mass exodus among the disgruntled home support who’d earlier had to suffer seeing goals in either half from Thomas O’Ware and Scott Tiffoney put the free-flowing visitors in complete command.
There are, of course, still 13 fixtures remaining but there are precious few signs to suggest they have either the ability or even stomach to attempt to rescue things accordingly.
For over an hour after the game, Laszlo was locked in the dressing-room telling his players some home truths which they more than deserved for their pitiful efforts.
Their previous encounter ended in a 2-0 win for United at Cappielow as recently as December when they found themselves in pole position in the race for promotion.
Morton, though, soon dictated and did have the ball in the net as early as the third minute when Gary Oliver headed home Frank Ross’ cross, but was deemed to be in an offisde position.
The hosts’ worst fears were realised in 35 minutes. Michael Tidser’s curling free-kick was left by the home defence and O’Ware rose to head downwards past Lewis.
It could have been even worse three minutes before the break. This time Ross saw his header from James Doyle’s cross gathered by Lewis at the second attempt.
Within two minutes of the restart, disaster struck for the hosts. Tiffoney shrugged off Paul Quinn before striking an unstoppable effort which dipped high over Lewis and into the roof of the net. Morton made it 3-0 in with ten minutes left. Iredale easily danced past four opponents before burying a sweet left-foot shot into the far corner.
Jim Duffy, the visiting manager, savoured the club’s first league win at Tannadice in 40 years.
“Every single player put in an inredible effort,” smiled Duffy afterwards.
“It’s some achievement for my players to come to a place like Tannadice and win so comprehensively, especially for the first time in a long while.”