United should get about 2,000 tickets for the last-16 second leg contest at the San Mames Stadium on March 15.
However, they have been stunned to discover Athletic intend to charge £77.50 for the tickets, a massive hike on their previous-round triumph over Lokomotiv Moscow.
Although it is common practice in Spain for vast disparities in prices to exist for certain games, United feel the sums being charged are excessive.
They have already expressed their concerns to Athletic, but received little sympathy on the grounds that the La Liga outfit intend to charge home supporters exactly the same.
So, whilst they privately accept there is little chance of the situation changing, Press Association Sport understands that United feel they must make their feelings known to UEFA too.
It will be the first time the Red Devils have visited the Basque stronghold of Spain since January 1957, when they lost 5-3 in a European Cup quarter-final, only to turn the tie round at Maine Road in the second leg.