Aviation officials are investigating an incident in which an aircraft flew over Preston North End FC's Deepdale ground with a banner mocking their relegation.
Two separate messages appeared in the sky above the stadium during the team's 3-1 victory over Watford on Saturday.
One read: "Poor little Preston enjoy Lge 1" while the other boasted: "We are superior love Blackpool FC."
The Civil Aviation Authority (CAA) said no permission was granted and is now examining if safety rules were broken.
North End's relegation from the Championship was confirmed last month after a 1-0 defeat to Cardiff.
The light aircraft appeared and circled above the ground during their hard-fought win, which lifted Preston off the foot of the table.
The stunt provoked anger from manager Phil Brown, who said he wanted to shoot the plane down.
Although it has not received a formal complaint, the CAA is examining pictures to determine whether the unidentified pilot broke safety rules.
If the authority does decide to prosecute and the pilot is found guilty of infringements, he or she could be jailed or fined.
"The key rule is that aircraft cannot fly below 1,000 ft over a built-up area. So if we find out that has happened we can take legal action," said a spokesman.
"Any breach of navigation orders we take very seriously and will take legal action.
"We need to protect people on the ground and other airspace users."