Livi hurt Airdrie's hopes
Airdrie's play-off hopes were dented after Ian Russell's early double helped champions Livingston win 4-2 at New Broomfield.
Russell easily headed home his first goal after five minutes from Callum Fordyce's cross.
His second came just two minutes later when he fired into the bottom corner from just outside the penalty area.
United pulled one back 13 minutes before half-time, Paul Lovering providing the close-range headed finish from Graeme Owens' free-kick.
And they equalised 55 minutes in, Jamie Stevenson crossing for Owens to slot in from inside the box.
But Raffaele De Vita's close-range shot from David Sinclair's centre restored the away side's lead within six minutes and young Marc McNulty secured the victory with a long-range effort 10 minutes from time.