Pompey surrendered a 1-0 lead as their dreams of lifting the FA Youth Cup were crushed by Swansea tonight.
The Welsh side won the right to host Manchester United in round five after a fortuitous 2-1 triumph at Fratton Park.
Alex Grant had fired the young Blues into a second-half lead only for the Swans to hit back seconds later.
And then the visitors snatched victory with a freak strike on 80 minutes – as a cross flew over the head of goalkeeper Matt Gledhill and into the net.
Pompey earlier named new signing Dan Thompson on the bench, while Swansea were unchanged from the side that beat Liverpool 3-1 in the third round.
The Swans opened well and Sam Magri had to be at his best to deny Alex Bray an early sight of goal.
Gradually the young Blues picked up the pace and they soon threatened with Ryan Williams and Elliott Wheeler causing havoc on the flanks.
On 15 minutes, Williams cut loose and fired in a shot Swansea keeper Oliver Davies did well to turn behind.
He then linked up with Wheeler to tee up George Colson whose header flashed the wrong side of the post.
Midway through the half, the Welsh visitors lost their skipper Josh Pike to an ankle injury. And Williams was soon making a nuisance of himself again, crossing for Ashley Harris who could only blaze wide from close range.
Harris then failed to hit the target with a free-kick after a trip on Jack Maloney.
Then – just before the break – Pompey keeper Matt Gledhill came to the rescue when he somehow turned over Henry Jones’ powerful header from point-blank range.
On 50 minutes, Pompey took charge again and were unlucky not to take the lead when an inswinging corner was deflected goalwards before being hacked off the line.
But moments later, Andy Awford’s young Blues deservedly broke the deadlock.
Butler’s corner was headed back across goal by Colson and Grant bullied his way in to blast the loose ball home.
However, their lead lasted barely 60 seconds as Jones stepped up to thunder home a stunning free-kick from 30 yards. Colson had a great chance to restore the lead but lifted his hurried finish over the crossbar.
Thompson then came off the bench to add his aerial power up front.
But before he had chance to make an impact, Swansea snatched their winner as Loveridge’s cross soared over the head of Gledhill and into the net.
Pompey: Gledhill, Wheeler, Butler, Awford, Magri, Grant, Williams, Colson, Harris, Wallace, Maloney. Subs: Warren, Thompson, Higgins, Triggs, Branford
Swansea City: Davies, Shephard, Pike, Tancock, Atyeo, Jones, Zola, Sheehan, Loveridge, Williams, Bray. Subs: Clark, Hurrell, Waters, Roberts, Elsey