Arsenal have announced that they have launched a "full review" following last night's scenes of chaos at the Emirates during the side's game with FC Koln.
The Europa League tie was due to kick off at 8.05pm but was delayed by an hour as thousands of ticketless German fans attempted to gain access to the stadium.
Around 20,000 Koln supporters are believed to have travelled to London for the match - their first foray in Europe in 20 years - despite the club's official away allocation of just 2,900 tickets.
Furthermore, many Koln fans appeared to gain access to the 60,400-capacity ground by posing as Arsenal fans, with some even spotted in hospitality areas.
In a statement issued this morning, Arsenal said: "We worked in full consultation with police and UEFA officials on the night and in advance of the game and had taken extensive steps in advance to prevent tickets being sold to visiting supporters.
"This included a number of measures including ensuring no tickets were sold via general sale and that no red memberships purchased after the draw were able to be used to get tickets in the home end for this match. We also worked with our colleagues at Cologne to stop supporters travelling without match tickets.
"The 3,000 tickets issued to Cologne fans was in line with competition rules but it is clear many more visiting fans arrived, causing significant congestion and disturbance outside the stadium before kick-off. Many tickets were sold through touts and this is very disappointing.
"We have launched a full review into the circumstances surrounding the game and will ensure any lessons that can be learned are used in the future."