Leicester City were made to pay for their slack defending as hosts Napoli advanced in the Europa League at their expense.
The Foxes had started the evening in pole position to advance from Group C but will now drop into the Europa Conference League knockout play-offs in February.
A dramatic conclusion to the evening saw Leicester almost given a late reprieve because of events over 1,000 miles away in Poland, when a stoppage time penalty was awarded to Legia Warsaw in their fixture against Spartak Moscow.
But Tomas Pekhart's spot-kick was saved by Spartak goalkeeper Aleksandr Selikhov, to ensure his side progressed as group winners instead of slipping down to third.
Leicester manager Brendan Rodgers will be aware his side contributed heavily to their own downfall against an injury-ravaged Napoli side.
Adam Ounas' early goal set the tone for an error-strewn display, with Caglar Soyuncu's ill-advised pass beginning the passage of play that saw the Algeria winger register his first Napoli goal since April 2019.
Ryan Bertrand was at fault as Luciano Spalletti's side extended their lead, playing Andrea Petagna onside, with the forward in turn squaring for Eljif Elmas to tap home. But the Foxes responded in superb fashion with Jonny Evans driving in an immediate response after a free-kick was only partially cleared and Kiernan Dewsbury-Hall levelling matters with a superb left-foot volley from the edge of the penalty area.
However, another defensive lapse, from a Leicester side whose last clean sheet came on the opening day of the Premier League season, proved decisive.
Bertrand, Harvey Barnes and Timothy Castagne were all guilty of not marking tightly enough as Giovanni di Lorenzo picked out Elmas, who allowed the ball to run across his body before driving it into the bottom right corner.
Familiar failings cost Foxes
Leicester travelled to Italy in the knowledge that several permutations, including a defeat, could have seen them advance, while Napoli knew that only a win would suffice.
And while Napoli shook off some patchy Serie A form to achieve their objective, Leicester's struggle for consistency, and the passive defending which has underlined an underwhelming campaign so far, continued.
On paper the Foxes' strong starting XI, including the likes of Youri Tielemans and Jamie Vardy, looked more than capable of having the measure of a side shorn of Kalidou Koulibaly, Victor Osimhen and Lorenzo Insigne amongst others.
But there was little evidence of that on the pitch, bar a 20-minute period at the end of the first half when they recovered from a two-goal deficit.
While James Maddison struck a post and Evans emerged as the only Leicester defender worthy of credit, they have now conceded two goals or more in five of their past six matches and worryingly also have a combination of Covid-19 cases and illness in the squad to contend with.
Player of the match
ManolasKostas Manolas
Napoli
Squad number44Player nameManolas
Squad number7Player nameElmas
Squad number33Player nameOunas
Squad number37Player namePetagna
Squad number2Player nameMalcuit
Squad number5Player nameJuan Jesus
Squad number14Player nameMertens
Squad number13Player nameRrahmani
Squad number20Player nameZielinski
Squad number11Player nameLozano
Squad number4Player nameDemme
Squad number6Player nameMário Rui
Squad number22Player nameDi Lorenzo
Squad number1Player nameMeret
Leicester City
Squad number22Player nameDewsbury-Hall
Squad number6Player nameEvans
Squad number10Player nameMaddison
Squad number29Player nameDaka
Squad number7Player nameBarnes
Squad number9Player nameVardy
Squad number8Player nameTielemans
Squad number25Player nameNdidi
Squad number1Player nameSchmeichel
Squad number11Player nameAlbrighton
Squad number27Player nameCastagne
Squad number4Player nameSöyüncü
Squad number42Player nameSoumaré
Squad number5Player nameBertrand
Line-ups
Napoli
Formation 4-2-3-1
1Meret
22Di Lorenzo13Rrahmani5Nunes Jesus6Mário Rui
4Demme20Zielinski
11Lozano33Ounas7Elmas
37Petagna
1Meret22Di Lorenzo13Rrahmani5Nunes Jesus6Mário Rui4DemmeBooked at 77minsSubstituted forManolasat 78'minutes20Zielinski11LozanoSubstituted forMalcuitat 45'minutes33OunasSubstituted forMertensat 63'minutes7Elmas37PetagnaBooked at 33minsSubstitutes
2Malcuit14Mertens21Politano25Ospina44Manolas72Boffelli74Costanzo82VergaraLeicester
Formation 4-2-3-1
1Schmeichel
27Castagne6Evans4Söyüncü5Bertrand
8Tielemans25Ndidi
10Maddison22Dewsbury-Hall7Barnes
9Vardy
1Schmeichel27Castagne6Evans4Söyüncü5Bertrand8TielemansSubstituted forSoumaréat 77'minutes25Ndidi10Maddison22Dewsbury-HallSubstituted forAlbrightonat 89'minutes7BarnesSubstituted forDakaat 72'minutes9VardySubstitutes
11Albrighton12Ward20Choudhury29Daka33Thomas41Stolarczyk42Soumaré45Nelson49McAteer53EwingReferee:Antonio Miguel Mateu LahozLive Text
Match ends, Napoli 3, Leicester City 2.
Second Half ends, Napoli 3, Leicester City 2.
Patson Daka (Leicester City) wins a free kick in the attacking half.
Foul by Juan Jesus (Napoli).
Foul by Çaglar Söyüncü (Leicester City).
Andrea Petagna (Napoli) wins a free kick in the attacking half.
Attempt missed. Jamie Vardy (Leicester City) header from the centre of the box is too high. Assisted by Marc Albrighton with a cross.
Foul by Çaglar Söyüncü (Leicester City).
Andrea Petagna (Napoli) wins a free kick in the defensive half.
Substitution, Leicester City. Marc Albrighton replaces Kiernan Dewsbury-Hall.
Offside, Napoli. Juan Jesus tries a through ball, but Mário Rui is caught offside.
Corner, Napoli. Conceded by Kasper Schmeichel.
Attempt saved. Kevin Malcuit (Napoli) right footed shot from the right side of the box is saved in the top right corner. Assisted by Dries Mertens.
Corner, Leicester City. Conceded by Mário Rui.
Timothy Castagne (Leicester City) wins a free kick on the right wing.
Foul by Mário Rui (Napoli).
Jamie Vardy (Leicester City) wins a free kick on the left wing.
Foul by Kostas Manolas (Napoli).
Foul by Wilfred Ndidi (Leicester City).
Giovanni Di Lorenzo (Napoli) wins a free kick in the defensive half.