Chelsea fans have been asked to submit evidence to support concerns over their safety at Thursday's Conference League semi-final after Djurgarden fans had tickets for home sections of Stamford Bridge.
Videos shared widely on social media showed hundreds of supporters of the Swedish team stood in home areas of the stadium during the second leg which Chelsea won 1-0 to complete a 5-1 aggregate victory.
Some Djurgarden fans climbed over partitions to access the away section.
"This is the most serious breach of stadium security in recent memory, and it significantly undermines any security protocols Chelsea FC put in place," said the Chelsea Supporters' Trust in a statement.