Those who feel that Celtic have gone backwards since their Champions League heroics last year were offered further substance by the club’s worst ever European defeat, sustained at the Nou Camp on Wednesday night.

In the venue where they pushed Barcelona to the last kick of stoppage time in last season’s group stage, Neil Lennon’s players were long out of contention by the interval, following a half when Neymar displayed the attractive side of his game with a masterclass in stepovers, flicks and explosive bursts of pace to leave his markers for dead time and time again.

He scored his first tournament goal a few seconds before half-time and his second soon after the restart,

having previously supplied Pedro with a killer pass for Barça’s second, to supplement the opener from ­Gerard Piqué, scored after only seven ­minutes. Neymar’s third was not long delayed as Barça toyed with the ­Scottish Premiership leaders.

Lennon played a joker by opting to field Teemu Pukki, whose contribution had been underwhelming since joining from Schalke 04 in the ­summer. The Finnish striker was one of five changes from the side that won 5-0 at Motherwell on Friday as Nir Biton, like Pukki, made his first Champions League start, and Adam Matthews reappeared after a seven- week absence.

All of which confined Kris Commons, Charlie Mulgrew and Anthony Stokes to the bench where they could at least glance along and see that Andrés Iniesta and Jordi Alba were similarly kicking their heels, having been held as reinforcements for a side that lacked Lionel Messi.

Neymar, though, faced Scott Brown, back from a three-match suspension imposed after the teams first met in Glasgow when he flicked his boot at the Brazilian as the forward lay on the ground after being fouled.

Brown was seen often in the early stages, acting as runner between midfield and Pukki but the more significant action was evident along the Barça right flank, where Matthews found his match fitness tested in ­conditions of the utmost rigour.

Celtic took their illustrious opponents to the wire at the Nou Camp in 2012 and beat them in Glasgow by depriving them of the oxygen of space in the danger areas, but Fraser Forster had been exposed before Barcelona inflicted the first blow which started with Pedro prompting Alexis Sánchez for a drive which the goalkeeper blocked with his legs, only for the rebound to fall to the marauding Piqué, who struck home from close range.

To Celtic’s credit, they kept the issue alive for most of the first period, prompting the thought that they might make it into the dressing room but that slim hope was extinguished six minutes before the break when Piqué found Neymar for a cutback that begged to be put away – and duly was – by Pedro.

As Celtic visibly shrank, Neymar plunged the knife home yet again, getting his name into the tournament scoresheet with a smart shot from Martín Montoya’s prompt.

There was no relief from the ­Spanish Inquisition after the restart. Xavi extended the torment by picking out Neymar who struck a glorious drive from the edge of the Celtic box to leave Forster watching contrails as it swept beyond him.

The keeper was entitled to feel like the target in a shooting gallery as he first turned a Xavi drive over the top before being obliged yet again to retrieve the ball from his net when Neymar breezed past Efe Ambrose to shoot home via a deflection from Matthews.

Neymar would have got a fourth within moments – although he was offside when he took possession – after gallivanting around Forster only for Pedro to take the ball of his foot and drive off Ambrose. Cristian Tello, meanwhile, had replaced Alexis and, after being flagged offside at his first touch, delivered a rasping left-foot finish, with the help of a deflection off Virgil van Dijk, to take the score to a scarifying 6-0.

There was still time for that wholly unforeseen development, a Celtic goal headed home by Georgios Samaras from a free-kick by Mulgrew – who had replaced the hapless Pukki at half-time – but there was not a vestige of celebration by Celtic, who now know what it feels like to be on the wrong end of the sort of beating they have administered to Hearts and Motherwell lately.

Lennon said: “Barcelona were fantastic as we expected them to be but we were very poor. There was no bravery on the ball - we started off nervously and it looked like our heads went down at 2-0. We were weak and that has not been like us. It is a sore one and I’m hurting a bit, as are the player.”