The scoreline arguably flattered the hosts after two goals in the final two minutes but United will need to show a massive improvement in order to maintain their promotion push.
On a bitterly cold afternoon at the Shooting Field, United started looking to maintain possession and break down their stubborn opponents and could have been ahead within the opening five minutes when Pete Cooper's perfectly weighted pass was met by the run of Simon Catt
whose effort beat Steyning 'keeper Matt Evans, but was cleared off the line by a covering defender.
The hosts took the lead against the run of play on twelve minutes as an angled pass split the United defence and found an on-running Craig Knowles who despatched his shot past Adam Sargent
despite appeals for offside from United.
United, despite being a goal down, had more play than their opponents with Gianluca Del-Gaudio
and Jack Divall
looking the most likely to create opportunities down either wing.
United spurned numerous half-chances in the first half with Cooper, Catt and Del-Gaudio going close before Liam Baitup
had the best chance of all, but the 26-goal striker screwed his shot wide.
The second half brought saw Steyning come back into the game and attempt to keep the ball away from United, but Divall found space to produce a driving run into the area before teeing up Pete Featherstone whose strike cannoned back off the crossbar.
The sucker punch for United came on 61 minutes as a Sargent clearance fell straight to the feet of Knowles, who immediately returned the ball over the ‘keepers net and put the hosts into a 2-goal lead.
With United needing goals in a hurry, Paul Daubeney changed formation to 3-5-2 with Jamie Bunn
and Nick Barden
brought on to replace the tiring legs of Divall and Del-Gaudio.
The change brought more freedom for Featherstone in the middle of the park alongside Catt and Steve Stracey
, with the United captain forcing Evans into a full length save with ten minutes remaining.
With United pushing forward, the defence became exposed and Ben Hughes
failed to clear his lines sufficiently as Phil Williamson pounced to drive a shot past Sargent and kill off the contest.
Unfortunately for United, there was still time left in the contest for a fourth Steyning goal as Knowles completed his hat-trick on the stroke of full time with a shot that was spilled into the net by Sargent.
The defeat leaves United in 3rd place in SCFL Division Two on 45 points, 10 points behind leaders Newhaven with three games in hand and United scheduled to visit Fort Road at the end of March.
EUAFC: Sargent; Davidson, Lee (Hughes 78), Sherriff, Featherstone (C), Stracey, Del-Gaudio (Bunn 70), Catt, Cooper, Baitup, Divall (Barden 70)