Pompey were dumped out of the Papa John’s Trophy for the first time in three years after being convincingly beaten at Peterborough.
Goals from Jack Taylor, Jonson Clarke-Harris, Frankie Kent, Siriki Dembele and Ethan Hamilton saw the hosts progress to the quarter-finals.
There was at least a fine stoppage-time consolation for Ellis Harrison to restore some pride at the death.
But while the Blues selected a mixture of fringe players and youngsters, their opponents were almost at full strength.
Kenny Jackett retained just one starter from the narrow FA Cup defeat at Bristol City a couple of days earlier.
Rasmus Nicolaisen kept his place in the side, featuring in a defence that also included Paul Downing and James Bolton in front of debutant keeper Duncan Turnbull.
Teenagers Harry Kavanagh, Harvey Rew and Harry Jewitt-White joined Ben Close and Bryn Morris in midfield, while Harrison partnered Jordy Hiwula up front.
Peterborough, meanwhile, named a much more familiar looking line-up, which included former Blues players Nathan Thompson and Dan Butler.
But the visitors did not look over-awed in the early stages and almost took the lead with just four minutes on the clock.
Close swung a fine free-kick into the box for Harrison to meet at the back post and his header was blocked by Hamilton.
It was their opponents who did break the deadlock on 15 minutes, though, as the defence failed to cut-out Joe Ward’s low delivery and Taylor took advantage to drill into the bottom corner.
Pompey went close to levelling moments later as Bolton nodded on a Close corner and Hiwula’s acrobatic effort was tipped over the bar by Christy Pym.
The keeper was then called into action from the resulting flag-kick, this time keeping out a Jewitt-White header.
Turnbull came to the rescue at the other end, making a point-blank save to deny Siriki Dembele after the forward had stolen possession from Downing.
But he could do nothing to prevent Clarke-Harris doubling Posh’s lead with a fine free-kick after Downing had been penalised for a handball.
And they had a third goal just before the interval, as Ward’s delivery ricocheted off Kent and into the net.
Half Time: Peterborough 3 Pompey 0
It did not get any easier for the Blues when the action restarted and Peterborough soon had a fourth goal.
Butler sent a low cross into the area and Dembele was left in acres of space to turn the ball home from close range.
Jackett made a double switch just past the hour mark, with Taylor Seymour replacing Turnbull to become the second Pompey keeper to make his debut.
The other switch came in the centre of midfield, where Jewitt-White was replaced by fellow academy player Charlie Bell.
He was unable to take advantage on 69 minutes when a cross from Rew was cleared back into his path.
And although the visitors continued to work extremely hard against their more experienced hosts, they were struggling to create meaningful opportunities.
Harrison did win a free-kick in a dangerous position, although Close’s delivery was commandingly punched clear by Pym.
Alfie Bridgman then became the latest Pompey player to make his debut, with the youth team prospect replacing Rew for the closing stages.
But Posh wrapped things up late on, as a free-kick from Hamilton was drilled underneath the wall and nestled in the bottom corner.
There was still time for an impressive Pompey consolation, though, as HARRISON bent home a fine strike from outside the box.
Peterborough (3-4-1-2): Pym; Kent, Thompson, Beevers (c) (Mason 69); Ward (Kanu 46), Taylor, Hamilton, Butler (Burrows 63); Szmodics; Clarke-Harris (Eisa 57), Dembele (Jones 57)
Goals: Taylor 15, Clarke-Harris 29, Kent 45+1, Dembele 52, Hamilton 87
Subs not used: Gyollai, Brown
Pompey (3-5-2): Turnbull (Seymour 61); Bolton, Downing (c) (Johnson 72), Nicolaisen; Kavanagh, Jewitt-White (Bell 61), Close, Morris, Rew (Bridgman 81); Harrison, Hiwula
Goals: Harrison 90+2
Subs not used: Brook, Stanley, Marquis
Referee: Robert Lewis