The draw for the last 16 of the Champions League had to be "entirely redone" after an embarrassing technical error left clubs angry, European football's governing body UEFA said on Monday.
"Following a technical problem with the software of an external service provider that instructs the officials as to which teams are eligible to play each other, a material error occurred in the draw for the UEFA Champions League round of 16," a chastened UEFA announced.
"As a result of this, the draw has been declared void and will be entirely redone," at 1400 GMT.
With the draw being streamed live from UEFA's headquarters in Nyon, Switzerland, the problem was made evident when Manchester United's name was drawn to face Villarreal, despite the fact the two clubs could not have played one another having been in the same group.
Manchester City were drawn to play the Spanish side, but the ball containing United's name did not then appear to be replaced correctly and so they were not included as possible opponents for Atletico Madrid in the next tie.
Atletico, who were paired with six-time former winners Bayern Munich, said: "We are asking for an explanation from UEFA and we will see what they say."
United, meanwhile, were eventually drawn to play Paris Saint-Germain, meaning the mouthwatering prospect of Lionel Messi and Cristiano Ronaldo going up against each other again.
However, with the whole draw compromised, a red-faced UEFA was left with little choice but to start again from scratch.
The mess-up comes at a bad time for the European game's governing body, at the end of a year in which relations with some of the continent's biggest clubs were soured by the botched attempt at a breakaway 12-team Super League.
- 'Unbelievable' -
Among the Super League rebels were Real Madrid, who had been drawn to play Benfica in the first tie on Monday before the problems occurred.
"This is unbelievable," tweeted Michael Owen, the former Real Madrid and England striker who now works as a pundit.
A new draw done from scratch will leave some clubs with a bitter taste.
For example, holders Chelsea appeared to benefit from the original, flawed process when they were paired with French champions Lille, probably their weakest potential opponent, after qualifying for the last 16 in second place in their group.
Speaking at a press conference to preview his side's game with Leeds on Tuesday, City boss Pep Guardiola said: "I think it is fair. It was a mistake but sometimes these things can happen.
"If there was a mistake you have to repeat so it is not suspicious."
Clubs who were in the same group cannot play each other in the last 16, while teams from the same country are kept apart until the quarter-finals.
The last-16 first legs are due to be played in the second half of February with second legs in early March.
This season's Champions League final will be played in the Russian city of Saint Petersburg on May 28.
UEFA will soon introduce a new format for the Champions League group stage from 2024 which will see the number of clubs involved increase from 32 to 36 with each team guaranteed 10 group games compared to the current six.