Manchester United manager Sir Alex Ferguson rates Barcelona as the biggest threat to his side's chances of retaining their Champions League football title.
Both sides have managed to avoid one another in the quarter-finals, with United up against Porto and Barcelona facing Bayern Munich, and cannot now meet until the final in Rome at the end of May.
Also through to the last eight are United's English rivals Arsenal, Chelsea and Liverpool.
But, although Liverpool thrashed United 4-1 in the Premier League this month and saw off Real Madrid in the last 16 of the Champions League, Ferguson still believes Barcelona's the team to beat.
Barcelona won the Champions League in 2006 when then coach Frank Rijkaard put together a team renowned for its attacking football however Ferguson says current Barca boss Pep Guardiola has bolstered the Catalan club's defence.
The first legs of the Champions League quarter-finals are due to be played next week.