Italy, unlike Russia, has no such restrictions and thousands will make the easier journey to Rome without tickets.
United have an average attendance above 76,000, while Barcelona regularly attract crowds in excess of 90,000.
Chelsea's home gate is also double the ticket allocation for Rome but UEFA insist that it is their policy to make 75 per cent of the tickets available to the general public.
"The decision to split the tickets this way was taken a long time ago," a spokesman said.
"We have to allow football fans in general the opportunity to see the games and to thank the football family for their efforts over the season.
"We cannot say 100 per cent that more tickets won't be made available to fans of the two clubs but the capacity is what it is."
