Pompey started 2013 in depressing style as they slumped to a morale-sapping 5-0 defeat at Swindon.
The Blues turned in a fine first-half display as they got to the break 0-0 and looked like they might be good to earn a result on New Year’s Day.
But they capitulated in incredible style after the interval as substitute James Collins struck a remarkable four-goal haul - and he even set up the other for Danny Hollands.
Pompey looked good for an hour and were solid at the back before Swindon boss Paolo Di Canio turned to his third substitute, Collins, who turned the game for the home side.
The first came from a corner as he powered home a header and he made it 2-0 just a few minutes later with a low shot.
Hollands then cracked home to make it 3-0 before the superb Collins grabbed two more headers to rub salt in the wounds.
Pompey were left shell-shocked at the final whistle and now face the growing prospect of relegation from League One as they remain in the relegation zone.