Manchester United's memorial to mark the 50th anniversary of the Munich disaster, which features an image of the 1958 team which lost eight players in the air crash, has been attacked with paint bombs.
"A couple of vandals chucked paint bombs at the Munich tribute during the night and should be condemned for what they did," United spokesman Phil Townsend said. "I didn't think anybody would be so disreputable to do that."
Security will be increased ahead of the 6 February anniversary service at Old Trafford and the match against Manchester City four days later. The paint was aimed at the logo of United's shirt sponsor, AIG, whose presence on the tribute has enraged some fans.
"It is not an advert," Townsend said. "We are entirely comfortable that a key partner involved in the club would want to pay respects to the team on such an important anniversary."
The daubing is the latest controversy to overshadow preparations for the anniversary, with fears that the minute's silence before the Manchester derby will be disrupted by City fans.
The Irish Football Association, meanwhile, has confirmed that there will be a minute's silence before Northern Ireland's friendly against Bulgaria at Windsor Park on 6 February. Two Northern Ireland internationals were on board the plane – Harry Gregg and the late Jackie Blanchflower.