Mixu Paatelainen’s side were defeated by goals in either half from Miles Storey and Iain Vigurs and now sit eight points behind second-bottom Kilmarnock with only five league games left and needing a dramatic turnaround in fortunes to stay up.
United knew this was a massive afternoon as far as survival was concerned but still fell behind in 12 minutes to leave them with a major mountain to climb for the rest of the afternoon.
Caley Thistle’s Ross Draper beat the hosts’ offside trap before squaring to Storey who did the rest from close range to put the Highlanders on the front foot.
Inverness were firmly in command and in 23 minutes midfielder Greg Tansey hit a 20-yard shot which United keeper Eiji Kawashima saved at the expense of a corner.
Things got worse for the Tangerines in 38 minutes when an injury to Pau Dixon meant he had to be replaced by Simon Murray as they struggled badly to find any sort of rhythm.
Inverness remained on top after the break although United did threaten briefly in 47 minutes as Frenchman Guy Demel found space eight yards from goal when he hit a left-footed shot which was saved by Inverness keeper Owain Fon-Williams.
But their hopes of a comeback were dashed in 50 minutes when they fell further behind to leave their fans despondent.
United defender Callum Morris could headed his attempted clearance from Tansey’s corner straight to Vigurs who was standing 20 yards from goal.
Vigurs hit his shot well enough on his weaker right enough and Kawashima was not quick enough to react as the ball nestled low in his left corner.
That sparked a few departures among some home supporters who had seen enough of their favourites falling short once more.
They did threaten to pull one back in 65 minutes when youngster Kevin Knoyle got on the end of John Rankin’s cross, but Fon-Williams produced a brilliant reflex save.
From then on, Inverness did all they had to to see things out while their abject opponents can only really hope for a win in next weekend’s Scottish Cup semi-final with Hibs to give them some redemption.