Paris St Germain will face Real Madrid in a titanic clash in the last 16 of the Champions League while Manchester United take on Atletico Madrid after UEFA declared the original draw void due to a technical error in the process on Monday.

Earlier, United were mistakenly drawn with Villarreal, even though both teams had already faced each other in the group stage while the Premier League side's ball was not added to the bowl as a potential opponent for Atletico Madrid.

United were eventually drawn with PSG, Real drew Benfica, while Atletico drew Bayern Munich before the draw was scrapped after the Spanish side complained and UEFA admitted the error caused by a "technical problem with the software".

"It was surprising, unfortunate and very difficult to understand what happened... Millions of people were waiting for the draw," Real Madrid director Emilio Butragueno told the club's television channel.

"Having said that, we go into this tie with a lot of excitement... We're also aware of the difficulties, with the quality of players (PSG) have. We believe the team will play two great games, they'll be fantastic for football."

PSG's initial draw with United was a much anticipated battle between Cristiano Ronaldo and Lionel Messi -- the competition's two top goal scorers and winners of 12 of the last 13 Ballon d'Or awards, with Messi leading 7-5 after victory in 2021.

Instead, Messi will now face a familiar opponent in Real Madrid, against whom he has scored 26 times in all competitions when he was at Barcelona.

Real Madrid had also attempted to prise French forward Kylian Mbappe from PSG in the close season but ultimately failed after the Ligue 1 side rejected their bids.

United's Ronaldo, meanwhile, will relish a tie against Spanish champions Atletico having scored 25 goals against them for Real and Juventus.

'They had to do it again'

Liverpool take on Inter Milan with another trip to the San Siro after Juergen Klopp's side beat AC Milan twice in the group stage.

"I had to wait 54 years to play at the San Siro for the first time... and now it'll be twice in three months, so that's good news," Klopp told the Liverpool website.

"Of course, it is a tough draw, definitely. They are the league leaders in Italy; a good team in a good moment."

Klopp added UEFA did the right thing in redoing the draw.

"I saw it live and I thought, 'You cannot let it stand like this, there's no chance.' Definitely, they had to do it again."

Manchester City boss Pep Guardiola also agreed a redraw was the right course of action.

"I think it's fair," he said. "It was a mistake and these kind of things sometimes happen... (a redraw) makes sense so there are no suspicions."

Last year's runners-up City face Sporting Lisbon while Juventus, who topped their group above Chelsea, face Villarreal.

Holders Chelsea will face French champions Lille, just as they were drawn in the original draw, while six-times champions Bayern Munich will now face RB Salzburg -- the first Austrian side to qualify for the knockout stages.

Benfica will now play high-flying Ajax Amsterdam who impressed in the group stage by winning all six matches.

UEFA also experienced technical issues while conducting the third-tier Conference League draw later.

The botched process was reminiscent of the 1982 World Cup draw where then-FIFA general secretary Sepp Blatter attempted to prevent any of the four South American qualifiers from being placed in the same group.

However, FIFA did not stick to the procedure they had outlined and placed teams drawn in the wrong groups, leading to confusion among those assembled at the Madrid venue.

One of the spinning cages used to mix the balls also broke down during the redraw and caused further embarrassment.

Last-16 draw

Bayern Munich v RB Salzburg

Manchester City v Sporting Lisbon

Ajax Amsterdam v Benfica

Lille v Chelsea

Manchester United v Atletico Madrid

Juventus v Villarreal

Liverpool v Inter Milan

Real Madrid v Paris St Germain

Seeded teams (named first) will play the second leg at home.

First legs: Feb. 15-16 and Feb. 22-23

Second legs: March 8-9 and March 15-16