United will be playing their football in League One next season, after a 2-0 defeat at Crystal Palace.
Scunthorpe are nine points adrift of safety, with just three matches left, but do not have the 'R' next to their name just yet.
Leicester City, who sit fourth from bottom, have a much better goal difference than the Iron and still have to face Sheffield Wednesday, who are on the same points as them.
Due to this, the North Lincolnshire side's relegation will not be officially confirmed until Monday night at the earliest - when the Owls host Plymouth.
The hosts were comfortable winners in a poor game, and sealed their success in the first-half - thanks to goals from Tom Soares and Clinton Morrison.
United entered the match making just one change from the team which drew 1-1 at home to Sheffield Wednesday a week earlier.
Martin Paterson, fully fit after his struggle with gastroenteritis, returned to the side in place of winger Curtis Weston. The former Stoke man partnered Geoff Horsfield up front.
Scunthorpe - wearing all white - were backed by a loyal following of over 400 from North Lincolnshire.
The Iron started well, with Horsfield winning a free-kick on the edge of the area, but Grant McCann's subsequent set-piece from the left would not drop for a United player.
He then had another go from range five minutes later. However, his free-kick from the edge of the 18-yard box drifted just wide of Julian Speroni's top left-hand post.
Soares fired a speculative effort wide for the hosts on eight minutes and, four minutes later, Paterson created a great chance for himself - clipping the ball over Danny Butterfield, and advancing on goal before firing in a shot which Speroni comfortably gathered.
Palace took the lead on 21 minutes with their first shot on target. Soares got on the end of a low cross from Victor Moses on the left-hand side of the penalty area to send the ball past Joe Murphy from six yards.
Scunthorpe had started fairly well, but their lack of luck this season showed as the Eagles scored from their first real chance.
The pacy Moses had their next chance, firing in a low effort on the break which Murphy tipped out for a corner to his right.
Jonathan Forte had a decent chance to level the scores, but he could only head Ian Morris' cross from the right into the side-netting.
Palace then extended their lead on 38 minutes when Clinton Morrison headed in Moses' cross from point-blank range. He was booked for his celebration, in which he took his shirt off, but he needn't care - his team were well on course for three big points in the push for promotion.
And that is how it stayed until half-time.
After a string of corners, Scunthorpe had their first chance of the second period on 52 minutes when Andy Butler tamely headed wide from a Morris cross.
Ben Watson drilled an effort wide from distance for Palace three minutes later as they threatened to extend their advantage.
There wasn't much life in the game, with neither side looking like scoring.
With the game drifting away from the visitors, Paul Hayes came on for McCann as Scunthorpe increased their number of strikers on the pitch to four.
Although they had a fair bit of possession in the visitors' half, United still could not create anything of substance.
Palace created the best opening of the second period so far on 70 minutes when Scott Sinclair, on loan from Chelsea, drilled an effort over the crossbar from just inside the penalty area.
Nigel Adkins made another change on 72 minutes when Weston came on for Horsfield, as Scunthorpe tried to gain the width and creativity they had been lacking.
There was just one more chance with Sean Scannell drilling wide with just three minutes left. His shot rocketed just past the post and into the advertisement hoardings.
Izzy Iriekpen and Butler could have notched late consolations but it wasn't to be, as United went down to defeat away from home once more. They are still to win on the road in 2008.
The Iron are next in action when they host Cardiff City on Saturday.
Attendance: 15,975 (411 away).
