A point against Swindon at Glanford Park would have made relegation mathematically impossible, but despite a defeat results elsewhere now make survival all but certain.
The Iron are nine points clear of the Robins - who now occupy the final relegation spot.
And with three games to go for both teams, it would take three heavy defeats for the Iron and three big wins for Swindon - as well as various other unlikely results - for other teams to overtake United for relegation to happen.
Swindon are the only team in the bottom four who can catch Scunthorpe, with Hartlepool, MK Dons and Walsall too far adrift.
The Saddlers look doomed, and dropped to the foot of the table with a 1-0 loss at Doncaster, with James Coppinger hitting the winning goal for Rovers.
MK Dons gave themselves hope of another miraculous escape - last season they beat Torquay on the final day of the season to stay up, and send the Gulls down - with a last-gasp winner against Hartlepool.
Scott Taylor scored after 89 minutes, to move them above Pools in the table.
The Dons are now three points away from safety, with Yeovil's 2-1 loss at Nottingham Forest leaving them just three points above the drop zone.
The Glovers took an early lead at the City Ground through Phil Jevons, but Nicky Southall got a second-half winner for Forest after Ian Breckin's equaliser.
As a result of that win Forest are now in the play-off positions, replacing Swansea - who lost 2-0 at home to Rotherham.