MONACO, Aug 24 (Reuters) - Holders Barcelona will play Chelsea for the third successive season after being placed together in the same opening-round group when the draw for the Champions League was made on Thursday.
Chelsea knocked Barcelona out 5-4 on aggregate at the first knockout round stage in 2004-05 but the Spanish giants exacted revenge by winning 3-2 on aggregate last season en route to winning the final against Arsenal in Paris in May.
The English champions' previous encounters with their Spanish rivals were tense, controversial affairs.
Chelsea manager Jose Mourinho claimed his Barcelona counterpart Frank Rijkaard had visited referee Anders Frisk's dressing room at halftime of the first leg of the 2005 tie.
Last season, Chelsea accused Barcelona's teenage prodigy Lionel Messi of acting after a challenge by defender Asier del Horno, who was sent off for the offence.
'It's friends reunited with Chelsea now that we are meeting them for a third year in a row,' said Barcelona's sporting director Txiki Begiristain.
Chelsea, desperate to make an impact in Europe at last, know they will face a tough task againt the likes of Ronaldinho and Samuel Eto'o.
'Obviously, we know Barcelona well,' said Chelsea chief executive Peter Kenyon.
'We have a lot of a respect them. They will always be a challenge for any team.'
Werder Bremen of Germany and Levski Sofia, the first Bulgarian team to take part in the competition proper, complete Group A.
Barcelona also faced Werder Bremen at the group stage last season winning 2-0 in Germany and 3-1 at home.
Arsenal were handed a tough draw against 2004 European champions Porto and CSKA Moscow, winners of the UEFA Cup in 2005. Hamburg SV, European champions in 1983, complete Group G.
Arsenal, famous for nearly-misses in European club soccer's premier event, will feature in the Champions League for the ninth consecutive season but have never met any of their Group G opponents in competitive soccer before.
'We have a magnificent stadium, probably one of the most beautiful in the world, and we want to put a winning team on top of if', said Arsenal vice-chairman David Dean.
'Arsenal's aim is always to compete at the highest level. That will be our goal this season again,' he added.
Manchester United will face Celtic in an all-British clash, with Benfica also in Group F. FC Copenhagen, who knocked Ajax Amsterdam out with a shock win in the third qualifying round, make their Champions League debut in the same group.
Although United and Celtic have long played in each other in friendlies, the latest a testimonial for Roy Keane as recently as May, they have never met in competition before.
'The atmosphere when we played them in May was electric, so I imagine it will be even more special when we meet them for points,' said United chief executive David Gill.
The teams meet in their opening match at Old Trafford on September 13.
United also have a long rivalry with Benfica famously beating them in the 1968 European Cup final, and met them in the group stage last year, winning 2-1 at Old Trafford but losing 2-1 in Lisbon, a result that cost them a place in the knockout round.
AC Milan, six-times winners of the European Cup, face Lille of France, AEK Athens and Anderlecht in Group H after UEFA sanctioned their participation despite their involvement in the Serie A match-fixing scandal.
Real Madrid, the record nine-times European champions, face Olympique Lyon, Dynamo Kiev and 1986 winners Steaua Bucharest in Group E.
Lyon will start their campaign by hosting Real Madrid, whom they had stunned 3-0 at Gerland last season.
The five-times French champions will meet a familiar face in Mali midfielder Mahamadou Diarra, who has just left them for Real.
Liverpool who secured a stunning penalty shootout victory over AC Milan to win the European Cup in 2005 in Istanbul return to the Turkish city to face Galatasaray in Group C which also includes PSV Eindhoven and Girondins Bordeaux.