John Barnes's first season as Celtic head coach was in ruins last night after Inverness Caledonian Thistle inflicted one of the most stunning shocks in this competition's history.
Going into the match trailing Rangers by 10 points in the Scottish Premier League, Celtic needed to launch a morale-boosting cup run but instead fell dreadfully at this first hurdle.
For the already under-fire Barnes it was the worst case scenario made real as First Division Inverness took the game to the Glasgow giants and humbled them with relative ease.
Barry Wilson's 16th-minute header was cancelled out just two minutes later by Mark Burchill, but that was to prove Celtic's last reprieve as the visitors regrouped and came again.
On 24 minutes Lubomir Moravcik put through his net for a lead Inverness held to the interval, and then 12 minutes after the break Paul Sheerin converted a match-sealing penalty.
The astonishing reverse now seems certain to place massive question marks over Barnes's position and that of the man who appointed him, director of football operations Kenny Dalglish.
Celtic were without Paul Lambert and Alan Stubbs through injury, but did reintroduce Olivier Tebily after his traumatic international duty with the Ivory Coast.
Burchill earned a rare start in attack, and Celtic started brightly when Jim Calder was forced to grasp a fierce Moravcik effort at the second attempt.
Inverness moved ahead with their first attack of note when Sheerin's cross from the left was headed into the roof of the net by Wilson to delight the 4,000-strong travelling support.
Their initial joy was to be short-lived because just two minutes after falling behind Celtic were level when Berkovic slid Burchill through.
The Scotland international at first tried to find Mark Viduka. But a break of the ball gave him a second chance, and he found the time to drill low past Calder.
That looked to signal the end of Inverness's hopes of a shock, but the speed of Celtic's response was to prove misleading as they continued to lack composure at the back.
Inverness were lively on the break, and Jonathan Gould needed to be alert to stop Michael Teasdale scoring via a close-range header on 22 minutes.
The warning went unheeded, and just two minutes later Celtic were behind a second time as Bobby Mann's header was deflected beyond Gould by Moravcik.
The jeers echoed around the stadium as Celtic's frayed nerves made clear an upset was on.
Barnes made a bold switch at half-time as he pulled off leading scorer Viduka and brought on former England international Ian Wright to partner Burchill.
Celtic's plight worsened on 56 minutes when a needless push on Wilson by Regi Blinker produced a penalty that Sheerin converted by sending Gould the wrong way.
Celtic had to give everything. But to their fans' fury they seemed to have very little to offer, Tebily's over-ambitious shot from 30 yards summing up their lack of invention.
CELTIC: Gould, Boyd, Tebily, Mahe, Riseth, Healy, Blinker, Moravcik, Berkovic, Viduka (Wright 46), Burchill. Subs Not Used: Kerr, Petta. Booked: Tebily, Wright, Healy. Goals: Burchill 17.
INVERNESS CT: Calder, Teasdale, Golabek, Mann, McCulloch, Hastings, Sheerin, Tokely (Byers 90), Wilson (Glancy 87), Christie, Wyness (Bavidge 79). Booked: Tokely. Goals: Wilson 16, Moravcik 24 og, Sheerin 57 pen.
Referee: D McDonald (Scotland).
