ALTRINCHAM slumped to their fifth defeat in six matches on Sunday with this loss to a team from a step lower down in the non-League pyramid.
Victory would have won the club £4,000 as well as progress to the next round of the FA Trophy.
After a reasonably bright start from Altrincham, United gradually got on top.
Their main attacking focus was their speedy left-winger and former Altrincham player, Carlos Roca. He put his side ahead just before half-time when the Rebels were given the benefit of the doubt about an offside decision. The ball came to Roca who fired into the top of the net from outside the box.
Having barely threatened a goal in the first half, Alty levelled the scores just seven minutes into the second half when Jack Redshaw, the shortest player on the pitch, was given a free header from a Nicky Clee corner from the right.
After this, with Alty having their best spell of the game, they made a couple of changes to their attack, as Twiss and Lawrie replaced Redshaw and Clee.
But it was FCUM who scored again, with 12 minutes remaining. This time Roca played a fine pass to another former Alty man, Astley Mulholland. His cross was headed home by centre-forward Mike Norton.
Despite a late flurry, in which Altrincham won four corners, FC United hung on to secure a tie in the first round proper of the FA Trophy.
ALTRINCHAM Coburn, Lees, Flynn, Williams (Watmore), Brown, Richman, Danylyk, Densmore (capt), Clee (Lawrie), Reeves, Redshaw (Twiss). Unused subs: Reid, Coombs