Mick McCarthy’s side began the afternoon in fifth place with a three-point cushion to seventh and could hardly have started better at Ewood Park with Daryl Murphy volleying them ahead after just two minutes.
But Jordan Rhodes and Craig Conway struck before half-time to put Blackburn 2-1 up and with results elsewhere, Ipswich dropped to sixth, hanging on courtesy of goal difference.
Rudy Gestede made it 3-1 from close range in the second half to shred the nerves of the visiting fans, whose side fought frantically to get back in the game.
They hit a second through Murphy’s late penalty – his 27th goal of the season – and, although they could not draw level, any worries were alleviated with a loud cheer from the visiting end signalled Derby’s capitulation.
Ipswich have only won two of their last 10 away from home but will not care after securing a mouth-watering play-off tie with arch-rivals Norwich.
David Dunn was handed his first league start of the season and the captain’s armband on his final Blackburn appearance, replacing the ineligible Paul Taylor in one of two changes from the draw at Huddersfield.
McCarthy recalled Jay Tabb in for Stephen Hunt who dropped to the bench.
The visitors made an ideal start when Tabb’s corner was ferociously volleyed home by Murphy in the second minute from 12 yards, giving Rovers goalkeeper David Raya no chance on his Ewood Park debut.
Freddie Sears hit the bar from a tight angle after 15 minutes but Rovers levelled in the 36th minute when Conway’s superb free-kick was headed home by Rhodes from six yards for his 21st goal of the season.
And Rovers turned the tables completely six minutes later when Gestede headed on a long throw and Conway ghosted in behind the Ipswich defence to volley into the bottom corner.
Early in the second half, Rhodes reacted quickest from close range to fire goalwards but Bartosz Bialkowski was in the right place to somehow prevent a third.
Things got worse for McCarthy’s men after 57 minutes when a clearance off Conway’s left-wing cross hit an Ipswich player and fell invitingly to Gestede who prodded home his 22nd of the campaign.
Sears then missed the easiest chance of the afternoon when Tyrone Mings’ header was parried by Raya to the forward two yards out and he somehow lifted it over the bar.
It could have been worse for Ipswich after 77 minutes but Josh King’s thunderous 20-yard half-volley crashed back off the bar and was cleared.
Murphy scored his and Ipswich’s second of the game from the spot eight minutes from time when Teddy Bishop was brought down by Tommy Spurr and though they could not equalise, Ipswich maintained their promotion hopes.