City lost 1-0 at Gillingham, with Dean McDonald's strike after 55 minutes ensuring the Iron's six point gap over the Robins was maintained.
Forest failed to take advantage of the top two losing, as they were held to a 1-1 draw at home by struggling Rotherham.
Chris O'Grady hammered the visitors into the lead with a stunning 20-yard drive after 10 minutes, but Grant Hold levelled from the penalty spot of Nathan Tyson went down in the area following a challenge by Craig Fleming.
Forest's point means they now trail Scunthorpe by 10 points, with a maximum of 12 still to be won.
Brentford were relegated to League Two following a 3-1 defeat at Crewe Alexandra.
Blackpool strengthened their play-off position, they're now fourth, after strikes from Claus Jorgensen and Robbie Williams earned them a 2-0 win at Huddersfield.
Yeovil came from behind to beat Leyton Orient 2-1, with the win moving them up to fifth.
Oldham needed a late equaliser from Luigi GLombard to rescue a point at Bradford City, after Moses Ashikodi had put the Bantams ahead. The Latics drop to sixth.
Carlisle kept up their play-off push with a 3-1 win over Bournemouth, while Swansea's 3-0 success over Port Vale keeps them hot on the heels of the top six.
Millwall beat Doncaster 2-1, Tranmere saw off Brighton by the same scoreline and Cheltenham and Chesterfield played out a goalless draw.