Celtic suffered a 5-1 extra-time defeat against LASK in northern Austria to crash out of Champions League qualifying despite holding a three-goal first-leg lead.
The Scottish club had won the first leg 4-1 in Glasgow six days earlier and extended their aggregate lead through captain Callum McGregor, but collapsed as the Austrian hosts scored five unanswered goals.
The catastrophic result denies Celtic a place in the Champions League main stage alongside an estimated £40million in prize money ahead of Thursday's group stage draw in Monaco.
Martin O'Neill's side opened the match brightly, with new signing Mika Baur crossing for Kasper Hogh before Baur intercepted a pass to set up Hogh for a shot saved comfortably by LASK goalkeeper Lukas Jungwirth.
LASK responded with long-range efforts and crosses, as Xavier Mbuyamba missed from close range at the back post following a flick-on and Melayro Bogarde hit the post via a deflection off McGregor.
Celtic broke the deadlock in the 21st minute through a fast move down the left flank, where Hyun-jun Yang linked up with Kieran Tierney before cutting the ball back to McGregor.
McGregor fired a left-footed strike that deflected off defender Andres Andrade and past Jungwirth to give Celtic a 1-0 lead on the night and a 4-0 advantage on aggregate.
Hogh was denied a second goal shortly after when a nudge in the back unbalanced him, while LASK forward Samuel Adeniran dragged a shot wide and committed a foul on Celtic defender Colby Donovan.
LASK response before half time
Celtic appeared in control until the closing minutes of the first half, when defender Cameron Carter-Vickers played a pass to Swedish midfielder Benjamin Nygren.
Nygren attempted a blind back-pass that was intercepted by Moses Usor, who finished calmly past goalkeeper Sinisalo to give the Austrians hope before the interval.
The home side struck again three minutes into the second half when Auston Trusty was beaten for pace and power on the left wing, allowing playmaker Robert Ljubicic to shoot through a crowd of players and beat Sinisalo.
Ten minutes later, a long pass caught the Celtic defense disjointed as Donovan yielded space to Ljubicic, who set up Adeniran to curl a left-footed strike past Sinisalo's outstretched arm and cut the aggregate lead to 4-3.
O'Neill responded by making changes, introducing Reo Hatate and Seb Tounekti before adding Luke McCowan and Liam Scales, while Dane Murray later replaced the injured Donovan.
Sinisalo turned Sascha Horvath's deflected shot wide and watched Joao Viktor Tornich head off target, before an Adeniran strike hit the bar and bounced off Sinisalo's back to stay out.
O'Neill received a yellow card from the referee for venting his frustration at officials as LASK dominated, completing regulation time with 31 total goal attempts while Kasper Jorgensen sent a shot narrowly wide.
Late equaliser and extra time collapse
LASK equalised on aggregate in the 90th minute when George Bello crossed from the left wing for substitute Florian Flecker, who headed in the rebound after Sinisalo saved his initial header.
Four minutes into the second period of extra time, McCowan was dispossessed on the edge of his own penalty area, allowing LASK to work the ball to Adeniran, who scored his second goal of the night to make it 5-1.
Sinisalo kept Celtic alive by saving a late penalty from Adeniran after Hatate fouled Krystof Danek, but the final whistle sealed Celtic's elimination.
Recruitment criticism and European history
The defeat highlighted growing frustration over Celtic's transfer business, after the board spent £30million on four summer signings while remaining in net profit for the window.
Two of those signings, Baur and Haissem Hassan, arrived too late in the transfer window to settle into the squad, while key positions including left-back, holding midfield, and creative attack were left unreinforced.
Celtic, who made their European debut 64 years ago in the Inter-City Fairs Cup, have now been eliminated in six consecutive Champions League qualification campaigns.
The loss marks the first time Celtic have surrendered a three-goal first-leg lead in Europe since falling to MTK Hungarian in the 1963-64 season.
LASK join AEK Athens, Cluj, Ferencvaros, Midtjylland, and Kairat Almaty on the list of teams to eliminate Celtic from Champions League qualifying in recent years.
