Celtic have sacked under-fire manager Tony Mowbray following Wednesday's 4-0 defeat at St Mirren.
Mowbray won 17 of the 30 league games played since he arrived from West Brom last June and struggled in Europe.
And the Glasgow club trail Scottish Premier League leaders Rangers by 10 points, having played two more matches than their Old Firm rivals.
Coach Neil Lennon will oversee first-team matters on an interim basis, with Kilmarnock visiting on Saturday.
Lennon played for Celtic between 2000 and 2007, captaining the club in his last two seasons. He returned as a coach in April 2008 and had been in charge of the Under-19 team during Mowbray's time in Glasgow.
The move to promote Lennon comes after assistant manager Mark Venus and first team coach Peter Grant departed along with Mowbray.
"Naturally I am very disappointed to be leaving Glasgow Celtic," Mowbray told the League Managers Association.
"I am very proud to have not only managed but also played for a club with such great tradition and that has tremendous roots in football history.
"I would like to offer my sincere thanks to all of the players and also to my staff who supported me so well. Finally, I would like to wish the club every success in the future."
A former defender at the club between 1991 and 1995, Mowbray succeeded Gordon Strachan to become the 16th manager of Celtic, with West Brom receiving £2m in compensation.
The affable figure had been one of three managers targeted, along with Burnley's Owen Coyle, now at Bolton, and Swansea's Roberto Martinez, now at Wigan.
On his arrival, he said he lived by a code of "honesty, integrity, humility and respect" and he hoped to bring those qualities to the Celtic team.
However, his reign was not a happy one, beginning with elimination from the Champions League, albeit against a slick Arsenal side, who have progressed with ease to the last eight of the tournament.
Dropping into the Europa League, Celtic were expected to qualify from a group that included Hamburg, Rapid Vienna and Hapoel Tel-Aviv, but they could only muster six points from six games, with just one win.
The domestic season started brightly and Celtic were four points ahead of Rangers going into the first Old Firm derby of the season on 4 October, but they lost 2-1 at Ibrox.
Highlights - St Mirren 4-0 Celtic
And away matches proved to be Mowbray Achilles heel, with Wednesday's 4-0 drubbing by St Mirren heralding Celtic's sixth league defeat on the road.
"This is a very sad day for everyone at Celtic," said chief executive Peter Lawwell.
"Tony is a very fine man and someone who I know is passionate about the club he served so well as a player.
"Clearly, we have had a difficult season and results have not been as we would have hoped.
"Tony is equally disappointed at some of our results this season but working so closely with him I know that throughout his period as manager he has always given the club his total and absolute commitment."
Strachan had won three titles in four seasons at Celtic Park but Mowbray quickly set about dismantling the squad he inherited, bringing in a raft of new faces.
The big money loan arrival of Spurs striker Robbie Keane in January briefly lifted spirits among supporters but Celtic could not make inroads into Rangers' lead.
Mowbray had a long career at Middlesbrough before his switch to Celtic and later moved on to Ipswich, where he started his coaching career.
Lennon will take charge on a game-by-game basis
He had a brief spell as caretaker boss at Portman Road after the sacking of former Scotland boss George Burley before becoming a manager in his own right at Hibernian.
Mowbray led the Edinburgh club to two top-four finishes in 2005 and 2006, after which he left for West Brom.
He led the Baggies to the Championship play-offs, losing to Derby, but the following season they won the title and promotion to the Premier League.
However, they only lasted a season in England's top tier and were relegated, despite receiving plaudits for their attacking style of football.
His summer switch to Celtic Park was greeted with widespread enthusiasm among the Celtic faithful but few of those will lament his departure.
"Tony is held in great affection by Celtic fans and it is obviously with great disappointment that we make today's statement," said chairman John Reid. "During his period here, Tony has always acted with great dignity as he has done today."
"We are all disappointed at the way things have gone this season. However, as supporters, we will collectively build again to re-establish the club to its pre-eminent position within Scottish football."
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