With the Belgian side already consigned to finishing bottom in Group E, all the pressure was on Carlo Ancelotti's men at Stadio San Paolo.

Yet any concerns for the hosts disappeared once Milik opened the scoring after just 125 seconds, capitalising on an error by teenage goalkeeper Maarten Vandevoordt, before adding two more before the break.

Dries Mertens netted Napoli's fourth from the penalty spot but Liverpool's win away in Salzburg means the Serie A side finish second in the group, behind last season's winners.

Vandevoordt became the youngest goalkeeper to start a Champions League game but the 17-year-old made a massive mistake to hand Napoli, who had already hit the bar through a Kalidou Koulibaly header, a third-minute lead.

Given an early touch of the ball by his defence, Vandevoordt tried to turn into space away from Mertens but succeeded only in presenting Milik with a tap-in.

The forward scored again in the 26th minute, converting a low right-wing cross from Giovanni Di Lorenzo, before completing his treble from the spot.

Vandevoordt was at fault again, bringing down Jose Callejon, and the teenager was sent the wrong way from 12 yards.

A one-sided opening 45 minutes allowed Napoli to ease off after the break, but they struck again from the spot when Casper De Norre was harshly penalised for handball.

Mertens stepped up to beat Vandevoordt with a Panenka, completing the youngster's humiliation.