Two goals from John Sutton ended Celtic's 17-match unbeaten run as Motherwell secured a deserved win over the Scottish Premier League leaders.
Sutton pounced on a mistake from Charlie Mulgrew to slide the home side into the lead after two minutes.
Darren Randolph saved well from Celtic captain Scott Brown before the break.
Shaun Hutchinson's header struck the bar before Emilio Izaguirre was ruled to have tripped Keith Lasley and Sutton scored from the 49th-minute penalty.
Suffering their first defeat since 10 November gives Celtic's title rivals, Rangers, the chance to reduce the gap to five points should they defeat St Johnstone later in the afternoon.
Ending a run of five straight defeats by Celtic - and 11 games without a win over the Glasgow side - moves Motherwell five points clear of sixth-placed Inverness Caledonian Thistle.
Motherwell had been forced to reorganise at the back, with Stevie Hammell coming in for the suspended Stephen Craigan.
Manager Stuart McCall shored up his midfield by introducing Ross Forbes, with striker Francis Jeffers dropped to the bench.
Celtic's changes were in midfield and attack, with Ki Sung-Yueng replacing the suspended Beram Kayal and Anthony Stokes preferred to Georgios Samaras up front.
However, it was a nightmare start by Charlie Mulgrew in defence that led to Motherwell's opener after only two minutes.
First, Chris Humphrey pounced on the central defender's fresh-air attempt at a clearance, but Daniel Majstorovic managed to block the winger's shot as he bore down on goal.
Mulgrew then hesitated while hoping that goalkeeper Fraser Forster would come out to clear a long ball over the defender's head.
Instead, Sutton poked the ball past Foster before driving it into the unguarded net.
Celtic pressed forward and Mark Wilson headed powerfully over, but it was Motherwell who were next to put the ball in the net after 18 minutes.
Sutton latched on to Gunning's long ball, rounded Forster and rolled it into the net only to be ruled offside.
Motherwell went close again when a Jamie Murphy cross evaded the head of Sutton and the unmarked Humphrey rushed his first-time shot at the back post, slicing well wide.
Celtic, though, now had the territorial advantage and came close to equalising during a spell of intense pressure.
Stokes broke clear inside the box, but his low shot was blocked by the foot of goalkeeper Darren Randolph, who then sent a clever 18-yard curling drive from Scott Brown round the post with his outstretched hand.
Motherwell remained dangerous on the break and Murphy sent Humphrey sprinting clear from the halfway line only to be pulled back a hasty referee's whistle for Majstorovic's foul on Lasley at the other end.
Celtic's increasing pressure almost paid off in the dying seconds of the first half, but Shaun Hutchinson cleared a Stokes shot from in front of his own goal.
However, Motherwell made another whirlwind start after the break, with a Hutchinson header following a Forbes corner striking the underside of the crossbar before being cleared.
The home side's second eventually came from the penalty spot, when Emilio Izaguirre clipped the heels of Lasley and Sutton sent Forster the wrong way with his spot kick into the roof of the net.
Celtic manager Neil Lennon immediately hauled off Stokes and the injured Mark Wilson and brought on Samaras and Efraim Juarez.
The change almost coincided with a goal for the visitors, but Randolph did superbly to get a strong hand to Kris Commons' powerful 25-yard free-kick that curled towards the bottom corner.
Celtic continued to press and Commons had another curling effort well saved after Sutton and Humphrey both had chances on the break.
However, the two-goal advantage was retained as Motherwell completed their first win at Fir Park over the Glasgow side since May 2005.
Sutton pounced on a mistake from Charlie Mulgrew to slide the home side into the lead after two minutes.
Darren Randolph saved well from Celtic captain Scott Brown before the break.
Shaun Hutchinson's header struck the bar before Emilio Izaguirre was ruled to have tripped Keith Lasley and Sutton scored from the 49th-minute penalty.
Suffering their first defeat since 10 November gives Celtic's title rivals, Rangers, the chance to reduce the gap to five points should they defeat St Johnstone later in the afternoon.
Ending a run of five straight defeats by Celtic - and 11 games without a win over the Glasgow side - moves Motherwell five points clear of sixth-placed Inverness Caledonian Thistle.
Motherwell had been forced to reorganise at the back, with Stevie Hammell coming in for the suspended Stephen Craigan.
Manager Stuart McCall shored up his midfield by introducing Ross Forbes, with striker Francis Jeffers dropped to the bench.
Celtic's changes were in midfield and attack, with Ki Sung-Yueng replacing the suspended Beram Kayal and Anthony Stokes preferred to Georgios Samaras up front.
However, it was a nightmare start by Charlie Mulgrew in defence that led to Motherwell's opener after only two minutes.
First, Chris Humphrey pounced on the central defender's fresh-air attempt at a clearance, but Daniel Majstorovic managed to block the winger's shot as he bore down on goal.
Mulgrew then hesitated while hoping that goalkeeper Fraser Forster would come out to clear a long ball over the defender's head.
Instead, Sutton poked the ball past Foster before driving it into the unguarded net.
Celtic pressed forward and Mark Wilson headed powerfully over, but it was Motherwell who were next to put the ball in the net after 18 minutes.
Sutton latched on to Gunning's long ball, rounded Forster and rolled it into the net only to be ruled offside.
Motherwell went close again when a Jamie Murphy cross evaded the head of Sutton and the unmarked Humphrey rushed his first-time shot at the back post, slicing well wide.
Celtic, though, now had the territorial advantage and came close to equalising during a spell of intense pressure.
Stokes broke clear inside the box, but his low shot was blocked by the foot of goalkeeper Darren Randolph, who then sent a clever 18-yard curling drive from Scott Brown round the post with his outstretched hand.
Motherwell remained dangerous on the break and Murphy sent Humphrey sprinting clear from the halfway line only to be pulled back a hasty referee's whistle for Majstorovic's foul on Lasley at the other end.
Celtic's increasing pressure almost paid off in the dying seconds of the first half, but Shaun Hutchinson cleared a Stokes shot from in front of his own goal.
However, Motherwell made another whirlwind start after the break, with a Hutchinson header following a Forbes corner striking the underside of the crossbar before being cleared.
The home side's second eventually came from the penalty spot, when Emilio Izaguirre clipped the heels of Lasley and Sutton sent Forster the wrong way with his spot kick into the roof of the net.
Celtic manager Neil Lennon immediately hauled off Stokes and the injured Mark Wilson and brought on Samaras and Efraim Juarez.
The change almost coincided with a goal for the visitors, but Randolph did superbly to get a strong hand to Kris Commons' powerful 25-yard free-kick that curled towards the bottom corner.
Celtic continued to press and Commons had another curling effort well saved after Sutton and Humphrey both had chances on the break.
However, the two-goal advantage was retained as Motherwell completed their first win at Fir Park over the Glasgow side since May 2005.