Match ends, Arsenal 1, Swansea City 2.
Arsenal's hopes of landing a first league title in 12 years suffered a heavy blow as Swansea came from behind to win at Emirates Stadium.
Joel Campbell slid to hook the Gunners into an early lead and they looked set to make up ground on leaders Leicester.
Swansea - missing boss Francesco Guidolin through illness - levelled when Wayne Routledge side-footed home.
Olivier Giroud and Alexis Sanchez struck the bar before Ashley Williams pounced to earn a crucial Swansea win.
The Welsh club now have a six-point cushion to the relegation zone but Arsenal's six-point deficit to Leicester City at the top of the table looks sure to yield ugly headlines.
Arsene Wenger's side have lost three successive games for the first time since 2010, severely damaging their title hopes in a season many regard as their best opportunity in recent times to be crowned champions.
After defeat at Old Trafford on Sunday, pundits lined up to criticise Arsenal's failings. Their shortcomings were again laid bare against the Swans as they missed chances and their soft centre again cost them dear.
Shortcomings - we have been here before
Swansea arrived in north London sporting six changes, with a teenage debutant in Stephen Kingsley at left-back and minus their manager, who is in hospital with a chest infection.
The early stages, in which the impressive Campbell - in for Theo Walcott - turned home Sanchez's pass, saw the visitors struggle but then cracks emerged again in Wenger's side.
Weak play by Mesut Ozil led to Jack Cork being allowed to create the equaliser and Petr Cech must be questioned for the missed punch that allowed Williams to turn in from a yard.
Former Manchester United defender Rio Ferdinand has said Arsenal keep failing to sign leaders and such analysis continues to look more and more astute as at no point did Arsenal pin their visitors back for a long spell.
There were inevitable boos and the same old questions will be trotted out. Where is the character? Where is the steel? Arsenal are in danger of failing and suddenly, even a top-four place does not look guaranteed.
Is it all over?
Sitting third - behind Tottenham - and six points adrift of Leicester, Arsenal may not look out of the title race by any stretch.
But a focus on their run-in reveals difficult away trips to Spurs, Everton, West Ham, Sunderland and Manchester City. Such challenges currently look beyond them and with just three wins in 11 matches, the excellent December form that solidified their challenge has disappeared.
Wenger admitted he was "worried" about results after this latest loss. Arsenal fans will likely echo his thoughts. News that Cech picked up an injury that rules him out of Saturday's trip to White Hart Lane will only add to their concerns.
Swans staying up?
There were moments where Swansea rode their luck, particularly when Giroud volleyed on to the bar before the break and when Sanchez curled a free-kick on to the woodwork.
But with coach Alan Curtis in charge on the night, they grew into the game and could have led late on when substitute Gylfi Sigurdsson rounded Cech but fired wide.
The fact they managed 11 shots in a difficult away fixture perhaps underlined the footing they had in the game, with Williams leading the resistance at the other end.
The club captain made a game-high nine clearances and, as the only survivor in Swansea's defence from the defeat at Tottenham, marshalled the back four expertly.
His late tap-in was perhaps deserved reward and another win against struggling Norwich on Saturday could see this become the week in which Swansea's top-flight status is all but secured.
Man of the match - Ashley Williams
The stats you need to know...
- This is the first time the Gunners have lost back-to-back Premier League matches since November 2014 - those two games were also against Swansea and Manchester United.
- Wayne Routledge scored his first Premier League goal of the season and his first since December 2014.
- Ashley Williams has scored two Premier League goals in 2015-16, the same number he had scored in his previous four Premier League seasons.
Arsenal face a north London derby at Tottenham on Saturday in a fixture that looks pivotal at the top of the table, while a few hours later Swansea host Norwich in a game that seems key at the bottom.
- 28CampbellSubstituted forWelbeckat 64'minutes
- 17SánchezSubstituted forWalcottat 75'minutes
- 8FerSubstituted forFultonat 71'minutes
- 10A AyewBooked at 40minsSubstituted forRangelat 90'minutes
- 4Ki Sung-yuengSubstituted forG Sigurdssonat 45'minutes
- 15RoutledgeBooked at 77mins
- 23G Sigurdsson
- Robert Madley
- Shots on Target
Second Half ends, Arsenal 1, Swansea City 2.
Attempt missed. Bafétimbi Gomis (Swansea City) right footed shot from more than 35 yards misses to the left. Assisted by Wayne Routledge.
Corner, Arsenal. Conceded by Stephen Kingsley.
Theo Walcott (Arsenal) wins a free kick on the right wing.
Foul by Wayne Routledge (Swansea City).
Attempt missed. Danny Welbeck (Arsenal) header from the centre of the box is close, but misses to the left. Assisted by Theo Walcott.
Foul by Olivier Giroud (Arsenal).
Jack Cork (Swansea City) wins a free kick in the defensive half.
Substitution, Swansea City. Angel Rangel replaces André Ayew.
Offside, Swansea City. André Ayew tries a through ball, but Bafétimbi Gomis is caught offside.
Attempt saved. Theo Walcott (Arsenal) header from the centre of the box is saved in the top centre of the goal. Assisted by Mesut Özil with a cross.
Corner, Arsenal. Conceded by Jay Fulton.
Corner, Arsenal. Conceded by Kyle Naughton.
Foul by Danny Welbeck (Arsenal).
André Ayew (Swansea City) wins a free kick on the right wing.
Foul by Olivier Giroud (Arsenal).
Jordi Amat (Swansea City) wins a free kick on the right wing.
Attempt missed. Nacho Monreal (Arsenal) left footed shot from the centre of the box is high and wide to the left. Assisted by Olivier Giroud.
Offside, Arsenal. Mesut Özil tries a through ball, but Nacho Monreal is caught offside.