On another afternoon of high drama in the Championship promotion race, the Clarets' play-off hopes took a major blow as David Nugent returned to Turf Moor to punish his former team-mates.
The ex-Clarets' loanee put Pompey ahead with an explosive second-half finish as Burnley's three-game winning run came to an untimely end.
Chris Eagles replied as he claimed a further addition to his catalogue of top-class goals to go clear at the top of the Clarets' scoring charts with his 15th of the campaign.
But despite another massive effort to produce a come-from-behind victory, a desperately negative Portsmouth side managed to keep Eddie Howe's men at bay and leave them needing maximum points and a helping hand from elsewhere in the final two games to keep their promotion dream alive.
What might have been a day of celebration had started with a presentation to mark Graham Alexander's 1,000 career appearance as the Clarets' club captain was given a guard of honour by both teams before kick-off.
With Alexander given a magnificent ovation from a big Turf Moor crowd, the stage was set for Burnley to continue their surge towards the play-offs.
But at the tail-end of a season that has never been straight-forward, perhaps it was inevitable that it wouldn't be plain-sailing.
In a nervy start when the effects of Saturday's exertions in coming from behind to win at Derby also looked to have a bearing, the football that had so clinically accounted for the Rams and Middlesbrough before that took a while to come.
It took until the 15th-minute for the Clarets to post their first threat when Jay Rodriguez headed a Chris Eagles' cross wide following a rapid counter-attack.
Eagles then volleyed wide from long range and Wade Elliott nodded a Jack Cork cross at goalkeeper Jamie Ashdown at full stretch.
But clear-cut chances were hard to come by and it took a superb driving run and shot from Chris McCann, making his first appearance at Turf Moor since September, 2009, to force Ashdown into his first serious save with almost half an hour gone.
Ashdown also blocked the follow-up from Danny Fox but bar a header from Nugent, which Brian Jensen comfortably saved on his 300th Burnley appearance, it was all Burnley.
McCann was an increasing influence and drove another effort wide before the first half ended with Pompey surviving a penalty appeal when Joel Ward stooped to block an Eagles header with the top of his arm.
The Clarets maintained the momentum after the break as they peppered the Pompey penalty area without being able to force the break-through.
And disaster struck 12 minutes into the second half delivered a potentially knock-out blow.
The striker had been a popular figure during his season-long stay with the Clarets but that rating among the home fans plummeted when he accepted a square pass from Kanu to drill a right-footed shot into the top corner from the just outside the penalty area.
Suddenly the Clarets' play-off hopes were even further back in the balance and Howe wasted no time in sending on Nathan Delfouneso in place of the tiring McCann to try and a get a goal.
And almost immediately the Clarets had two chances to draw level.
First, the outstanding Rodriguez threaded Elliott through for an effort which Ashdown half parried and then when Eagles crossed, Iwelumo couldn't beat the Pompey keeper with a downward header from six yards.
The Clarets were giving it everything and just when it looked as though tired minds and legs might finally deny them Eagles produced a piece of magic to equalise with 16 minutes to go.
Eagles and Tyrone Mears have torn defences apart several times this season and did so again as they swapped passes before Eagles hammered an angled left-footed drive which flew into the top corner - possibly, it emerged later, via the slightest of touches off the forehead of Elliott.
With Turf Moor coming alive again, Delfouneso shot at Ashdown from too tight an angle as Burnley tried to wind up for one last push for in the face of frustrating delaying tactics from a Pompey side that dawdled to re-start the game at every opportunity and earned both Ashdown and David Cotterill bookings for time-wasting as referee Carl Boyeson added five minutes of injury time.
Gambling everything on a winner inevitably lift gaps at the back and Nugent could have nicked a second goal in the 86th-minute when clean through.
But Jensen made an excellent smothering block and Burnley continued to pour forward in search of a priceless goal that just wouldn't come as Greg Halford headed a Ross Wallace cross off the line in the final twist of another nerve-shredding instalment of the Clarets' promotion-seeking tale.
Burnley: Jensen, Mears, Fox, Bikey (Wallace 87), Duff, Cork, McCann (Delfouneso 60), Eagles, Iwelumo (Thompson 68), Rodriguez, Elliott. Subs: Alexander, Carlisle, Bartley, Grant.
Portsmouth: Ashdown, Halford, Hreidarsson, Mokoena, De Laet (Dickinson 75), Ward, Mullins, Hogg (Webber 79), Nugent (Ciftci 90), Cotterill, Kanu. Subs: Flahavan.
Booked: Ashdown 56, Hreidarsson 65
Referee: Carl Boyeson