Ipswich came from behind to beat Charlton 2-1 and take control of the race to join Coventry in the Premier League next season.
Darnell Furlong's strike and a Jaden Philogene penalty took Kieran McKenna's men back above Millwall into second place on goal difference.
But the Tractor Boys have three games remaining to Millwall's two.
Defeat means Charlton are still not mathematically safe from relegation, but they hold a six-point cushion over Oxford with two games to go.
The Addicks took a shock lead after only 42 seconds through Greg Docherty.
Ipswich took a while to get into their stride but levelled on 36 minutes when Azor Matusiwa found Furlong 20 yards out and the fullback lashed his shot beyond Thomas Kaminski into the far corner.
Furlong also played a major role in Ipswich's second, 12 minutes after half-time, when he was sent tumbling in the area by Macaulay Gillesphey.
Philogene's spot-kick was actually pretty tame but it rolled under the body of the diving Kaminski and squirmed into the net.
Blackburn guaranteed survival with a 3-1 win at Sheffield United thanks to Japanese duo Yuki Ohashi and Ryoya Morishita.
Rovers needed victory to seal safety in their penultimate game of the season and Ohashi struck twice either side of Morishita's effort as the visitors stormed into a 3-0 half-time lead.
Middlesbrough edged past Sheffield Wednesday 1-0 to realistically secure their place in the play-offs.
Morgan Whittaker's winner ended a seven-game winless run for Boro, who move six points clear of seventh-placed Hull but with a vastly superior goal difference.
Birmingham beat Preston 2-1 to climb up to 10th.