Hearts inflicted more relegation worries on the Tannadice outfit after a comfortable victory against a dispirited United side.
Having gained good fortune from two controversial decisions, the Edinburgh side strolled throughout the remainder of the game after adding a third goal early in the second period.
After a dull opening, the first chance fell to Kevin McKenna in the eighth minute.
Following a melee in the box in which United had numerous opportunities to clear, the ball fell to the unmarked Canadian who blasted over the bar from six yards.
Three minutes later, a 25-yard shot from Craig Easton was magnificently tipped over by Roddy McKenzie.
It took another 12 minutes before the next chance came. Good work by Jim Hamilton freed Charlie Miller whose shot was only stopped by the toes of McKenzie.
Against the run of play, the visitors went ahead in the 23rd minute following a highly controversial decision by referee Charlie Richmond.
Astonishingly he awarded a foul against Jim Lauchlan when the defender clearly won the ball in a tussle.
He then ignored a push by Mark De Vries as the Hearts striker leaned on Jim Hamilton to loop his header in from 12 yards.
Not surprisingly, United captain Easton was booked for dissent as he tried to relay his frustration at the referee's Christmas present.
More controversy surrounded Hearts' second goal in the 41st minute.
With the United defence bellowing for two offside decisions after Jean Louis Valois had found De Vries and again when the latter sent Andy Kirk clear, the Hearts player slid the ball into the net from two yards.
The official failed to raise his flag as Kirk looked across in expectation.
Six minutes into the second half, it was game over when the visitors made it 3-0.
Fabulous play by Valois on the left flank left Jamie McCunnie trailing. He threaded the ball through to Steven Boyack who immediately squared for the on-rushing Kirk to slot home his second from six yards.
With United's stand-in central defence of Lauchlan and Kevin McGowne in total disarray, they continued to fuel a feeling of nervousness throughout the team.
The only surprise was that the Jam Tarts did not take advantage and add more to their tally.