Celtic manager Martin O'Neil was animated on the touch line as Celtic lost 5-1. Craig Williamson/SNS Group via Getty Images Celtic threw away a four-goal lead in Austria as they missed out on the Champions League for the second year running.
The Scottish champions looked to be cruising into the league stage when Callum McGregor put them 4-0 up on aggregate against LASK with a 21st-minute deflected strike.
However, they went on to endure one of their all-time European lows with a 5-1 defeat in Linz delivering a £20 million-plus blow.
"We were far too passive. We don't have any excuses," said Celtic captain McGregor.
The Hoops had other chances in the first half but the Austrians gave themselves a lifeline when Moses Usor pounced on a loose pass from Benjamin Nygren to level on the night in the 32nd minute.
The tie began to spiral out of Celtic's control early in the second half as Robert Ljubicic and Samuel Adeniran netted from outside the box.
Martin O'Neill's side looked like they would cling on but Florian Flecker nodded home from close range in stoppage-time to spark an additional 30 minutes.
Celtic had disappeared as an attacking force at half-time and Adeniran delivered the knockout blow that sends the Hoops into the Europa League.
The Hoops exited on penalties at the play-off stage last season after failing to sign a proven striker before drawing a blank against Kairat Almaty. Fans will this year question the decision to sell Arne Engels just ahead of the first leg after they lost control in midfield in Austria.
The quality of Celtic's finishing had been the difference in Glasgow but LASK created chances throughout and the second leg started in similar fashion.
Celtic looked to extend their lead. Mika Baur stabbed an early chance just wide and Kasper Hogh forced a couple of first-half saves from decent opportunities.
McGregor opened the scoring in between those chances when he drilled a first-time shot in off a defender after Yang Hyun-jun had played a one-two with Kieran Tierney.
The home side had exerted some pressure just before McGregor's goal and they levelled following careless play from Nygren.
The Austrian champions pulled a goal back three minutes after the restart when Usor out-muscled Auston Trusty and worked the ball inside for Ljubicic to drive home from 20 yards.