The Hatters conceded another late goal at Kenilworth Road to condemn them to a third league defeat of the season.
Portsmouth substitute Matt Tubbs pounced in the 90th minute to give the visitors all three points in an open, end-to-end affair.
It was Pompey that took the lead in the eighth minute when Gareth Evans rifled home from close range.
But the Town’s response was good, and the hosts levelled on the quarter-hour when Josh McQuoid was fouled in the box and Craig Mackail-Smith converted the resultant penalty.
Both sides went for victory in the second half with Cameron McGeehan shooting wide, but it looked as the though the Town would have to settle for a point.
However, the visitors had other ideas, and won it at the death when Tubbs tapped home after Elliot Justham had parried Conor Chaplin’s shot.
It was another harsh lesson for the Town, who have now conceded last-minute goals in all three of their home League 2 fixtures this season.
Showing three changes to the side that lost on penalties to Stoke in the week – with Cameron McGeehan, Craig Mackail-Smith and Ryan Hall selected – the Town fell behind with the first chance of the game. There was a touch of misfortune about it too, as Luke Wilkinson’s clearance hit the head of Scott Cuthbert and the loose ball was hammered home by Evans from six yards.
However, the Hatters responded immediately and the lively McQuoid earned the Town a chance to level when he was upended by Nigel Atangana. Up stepped Mackail-Smith to drill home a low spot-kick for his first goal at Kenilworth Road to make it 1-1.
That goal gave the Town confidence, and McGeehan wasn’t far away from firing them in front 10 minutes before the break when he ended a patient passing move with a well-hit shot from 20 yards that whistled wide.
As the break approached, both sides traded chances. Pompey had the best of them and the Hatters needed Elliot Justham to be at his best when keeping out a low drive from ex-Town loanee Jayden Stockley.
At the start of the second half the Town made a change by introducing Olly Lee for Mark O’Brien in central midfield.
And the Hatters spurned a great opportunity to take the lead 10 minutes after the restart when McGeehan and Mackail-Smith pinched the ball off Pompey feet to race clear on goal. It was three Town men to Pompey’s two, but McGeehan opted to go alone and his low shot fizzed agonisingly wide of the target.
Back came Pompey, and the Town needed Lee to be in the right place at the right time when he cleared Gary Roberts’ header off the line following a left-wing corner as the visitors pushed for the lead.
Half-chances continued to come and go for both sides. Luke Wilkinson headed over for the Town on 68 minutes while Roberts and Kyle Bennett skied over for the visitors.
As time ticked into the final 10 minutes, the Hatters weren’t far away from a decent chance when an on-target Cuthbert header at the back stick hit a Pompey player with nine minutes left on the clock.
But, as has been the case in the two previous home league games, there was a nasty sting in the tail for the Town in injury time as Pompey won it. Two subs combined: Chaplin’s left-foot effort was well-parried away by Justham and the loose fell at the feet of Tubbs who coolly side-footed home from eight yards to sink the Town.
Town: Justham; O’Donnell, Griffiths, Cuthbert, Wilkinson; O’Brien (sub Lee 46), Smith, McGeehan; Hall (sub Ruddock 82), Mackail-Smith (sub Marriott 71), McQuoid. Subs not used: Tyler, McCourt, McNulty, Green.
Attendance: 9,083, the best of the season so far, including 1,032 backing Pompey.