Pompey exited the FA Youth Cup after snatching defeat from the jaws of victory against a spirited Cambridge side at Fratton Park
They had twice taken the lead through Jamie Howell and Koby Mottoh, only for United to hit back with goals from Brandon Njoku and Kai Yearn.
Howell struck again for the hosts, only for Glenn McConnell to hit a stoppage-time leveller after his penalty had been saved.
The same thing happened again for McConnell to find the target early in the additional time and Yearn then netted to leave Cambridge cruising.
A fantastic finish from Mottoh gave Pompey hope of forcing a penalty shoot-out, but they could not find a last-gasp leveller.
Blues boss Danny Cowley was among the crowd to watch the defeat, joined in the South Stand by Marlon Pack and Michael Morrison.
Zesh Rehman named the same side that had thrashed Three Bridges to reach this stage of the competition earlier in the month.
It looked as though they had fallen behind in the opening stages when skipper Jaydyn Lott converted a corner, only for a reprieve to come in the form of an assistant’s raised flag.
And Pompey quickly went up the other end to take the lead themselves, as HOWELL raced onto Sam Folarin’s incisive pass and calmly slotted home.
United looked to hit back straight from the kick-off and Malachi Osei-Owusu denied McConnell before Njoku slammed the loose ball against the post.
They did level the tie midway through the first half, though, as Njoku made space for himself inside the box before bending home a fine finish.
The visitors were starting to dominate the contest and McConnell angled a shot over after Pompey had failed to get a corner clear.
But they fell behind for a second time on 33 minutes, with Josh Dockerill sending a lofted ball over the head of Tom Finch and MOTTOH racing in to make sure it crossed the line.
United came close to another equaliser before the break following an untimely Osei-Owusu slip, only for the keeper to make a brilliant recovery and grab the ball from the feet of George Hoddle.
Half Time: Pompey 2 Cambridge 1
The visitors had a chance to get back on level terms straight after the restart, with McConnell’s 20-yard strike rattling the crossbar.
And the same player tried his luck with another long-range effort soon after, although this one flew over.
Pompey managed to regain their composure and Gabriel Kamavuako attempted to catch out Finch at his near post, with the keeper just about managing to scramble the ball behind.
But it was Cambridge who were next to score just past the hour mark, as Yearn rose to nod home a Greg Sandiford delivery.
It was certainly proving to be an entertaining contest and Finch parried away Harvey Laidlaw’s cross before keeping out the fierce follow-up from Adam Payce.
Both sides were looking capable of adding to the scoring and Osei-Owusu got down to gather a low Njoku effort.
But it was the Blues who were next to strike, with Mottoh being picked out by Payce and sending the ball into the mixer for HOWELL to turn home his second goal of the evening.
It looked as though they would see out the closing stages, only for Josh Dockerill to bring down Hoddle and referee Jordan McRitchie to point to the spot.
Osei-Owusu did well to keep out the penalty from McConnell, but the striker was first to the loose ball and forced it over the line.
The referee initially seemed to disallow it for reasons unclear to anyone, although changed his mind following a chat with his assistant.
After 90 minutes: Pompey 3 Cambridge 3
There was a sense of déjà vu around Fratton Park when a challenge by Kevin Bosaka saw McRitchie point to the spot again just after the restart.
Amazingly, McConnell again saw his effort saved by Osei-Owusu only to once more win the race for the loose ball and score.
And with Pompey on the ropes, the visitors netted again soon after, as Amaru Kaunda teed-up Yearn for a simple finish.
The Blues kept fighting, though, and were rewarded for their efforts with a superb strike from the edge of the box by MOTTOH on 114 minutes.
It raised hopes among the Fratton faithful of more late drama, but Cambridge held on earn a trip to Colchester in round three.
Pompey (3-5-2): Osei-Owusu; Quarm (Bosaka 86), Spurway, Dockerill; Mottoh, Payce (c), Simpson (Hurst 106), Kamavuako (Aston 72), Laidlaw; Folarin, Howell (Murray 86)
Goals: Howell 8, 77, Mottoh 33, 114
Booked: Spurway, Murray
Subs not used: Smith, Fox
Cambridge (4-1-2-1-2): Finch; Sandiford (Chadwick 82), Lott (c), Ewens-Findlay, Winterbone (Tyler-Cowlin 91); Scales; Hoddle, Ismaili (Kaunda 46); Yearn; McConnell, Njoku (Carey-Evans 113)
Goals: Njoku 22, Yearn 62, 96, McConnell 90+1, 93
Subs not used: Briggs, Staszewski, Habib
Referee: Jordan McRitchie