Uefa have launched an investigation into why the turnstiles of Baku’s Olympic Stadium were opened halfway through the Europa League final between Chelsea and Arsenal to let fans in for free.

Vast areas of empty seats could be seen inside the Azerbaijan venue on Wednesday night where Chelsea beat their Premier League rivals 4-1 to claim a second Europa League titles.

But fans who made the journey to Baku to follow their team noticed that those areas started to fill up long after kick-off – which came around half past midnight local time.

Uefa claims that 51,370 supporters turned out for the Europa League final, some 10,000 short of the stadium’s reduced capacity for the final of 62,000 and 16,000 below the maximum capacity.

There was also criticism of the atmosphere inside the Olympic Stadium with fans a long way from the pitch and a lack of noise leading to plenty of criticism during the match, noticeably from television pundits Cesc Fabregas and Eidur Gudjohnsen, while fans reported hearing atmospheric noises being pumped out of the stadium’s tannoy.

Player ratings from Chelsea vs Arsenal Show all 22 1 /22 Player ratings from Chelsea vs Arsenal Player ratings from Chelsea vs Arsenal Petr Cech - 7 Arsenal's Czech goalkeeper Petr Cech catches the ball during warm up prior to the UEFA Europa League final football match between Chelsea FC and Arsenal FC at the Baku Olympic Stadium in Baku, Azerbaijian on May 29, 2019. (Photo by Alexander NEMENOV / AFP)ALEXANDER NEMENOV/AFP/Getty Images ALEXANDER NEMENOV AFP/Getty Images Player ratings from Chelsea vs Arsenal Sokratis Papastathopoulos - 6 Soccer Football - Europa League Final - Chelsea v Arsenal - Baku Olympic Stadium, Baku, Azerbaijan - May 29, 2019 Chelsea's Olivier Giroud reacts as Arsenal's Sokratis Papastathopoulos gestures REUTERS/Maxim Shemetov MAXIM SHEMETOV REUTERS Player ratings from Chelsea vs Arsenal Laurent Koscielny - 6 Soccer Football - Europa League Final - Chelsea v Arsenal - Baku Olympic Stadium, Baku, Azerbaijan - May 29, 2019 Arsenal's Laurent Koscielny in action with Chelsea's Olivier Giroud REUTERS/Maxim Shemetov MAXIM SHEMETOV REUTERS Player ratings from Chelsea vs Arsenal Nacho Monreal - 5 Soccer Football - Europa League Final - Chelsea v Arsenal - Baku Olympic Stadium, Baku, Azerbaijan - May 29, 2019 Arsenal's Nacho Monreal in action with Chelsea's Pedro REUTERS/Amr Abdallah Dalsh AMR ABDALLAH DALSH REUTERS Player ratings from Chelsea vs Arsenal Ainsley Maitland-Niles - 7 Arsenal's Ainsley Maitland-Niles, left, goes for the ball next to Chelsea's David Luiz during the Europa League Final soccer match between Arsenal and Chelsea at the Olympic stadium in Baku, Azerbaijan, Wednesday, May 29, 2019. (AP Photo/Luca Bruno) Luca Bruno AP Player ratings from Chelsea vs Arsenal Lucas Torreira - 5 Arsenal's Uruguayan midfielder Lucas Torreira (L) and Chelsea's Croatian midfielder Mateo Kovacic vie for the ball during the UEFA Europa League final football match between Chelsea FC and Arsenal FC at the Baku Olympic Stadium in Baku, Azerbaijian on May 29, 2019. (Photo by Yuri KADOBNOV / AFP)YURI KADOBNOV/AFP/Getty Images YURI KADOBNOV AFP/Getty Images Player ratings from Chelsea vs Arsenal Granit Xhaka - 6 Soccer Football - Europa League Final - Chelsea v Arsenal - Baku Olympic Stadium, Baku, Azerbaijan - May 29, 2019 Arsenal's Granit Xhaka lines up before the match REUTERS/Maxim Shemetov MAXIM SHEMETOV REUTERS Player ratings from Chelsea vs Arsenal Sead Kolasinac - 5 Soccer Football - Europa League Final - Chelsea v Arsenal - Baku Olympic Stadium, Baku, Azerbaijan - May 29, 2019 Chelsea's Cesar Azpilicueta in action with Arsenal's Sead Kolasinac REUTERS/Amr Abdallah Dalsh AMR ABDALLAH DALSH REUTERS Player ratings from Chelsea vs Arsenal Mesut Ozil - 5 BAKU, AZERBAIJAN - MAY 29: Mesut Ozil of Arsenal is challenged by Jorginho of Chelsea during the UEFA Europa League Final between Chelsea and Arsenal at Baku Olimpiya Stadionu on May 29, 2019 in Baku, Azerbaijan. (Photo by Francois Nel/Getty Images) Francois Nel Getty Images Player ratings from Chelsea vs Arsenal Alexandre Lacazette - 6 Soccer Football - Europa League Final - Chelsea v Arsenal - Baku Olympic Stadium, Baku, Azerbaijan - May 29, 2019 Arsenal's Alexandre Lacazette in action with Chelsea's Mateo Kovacic REUTERS/Amr Abdallah Dalsh AMR ABDALLAH DALSH REUTERS Player ratings from Chelsea vs Arsenal Pierrer-Emerick Aubameyang - 6 Arsenal's Gabonese striker Pierre-Emerick Aubameyang reacts during the UEFA Europa League final football match between Chelsea FC and Arsenal FC at the Baku Olympic Stadium in Baku, Azerbaijian, on May 29, 2019. (Photo by OZAN KOSE / AFP)OZAN KOSE/AFP/Getty Images OZAN KOSE AFP/Getty Images Player ratings from Chelsea vs Arsenal Kepa Arrizabalaga - 6 Chelsea's Spanish goalkeeper Kepa Arrizabalaga makes a save during the UEFA Europa League final football match between Chelsea FC and Arsenal FC at the Baku Olympic Stadium in Baku, Azerbaijian on May 29, 2019. (Photo by Yuri KADOBNOV / AFP)YURI KADOBNOV/AFP/Getty Images YURI KADOBNOV AFP/Getty Images Player ratings from Chelsea vs Arsenal Cesar Azpiulicueta - 7 Arsenal's Alexandre Lacazette, left, fights for the ball with Chelsea's Cesar Azpilicueta during the Europa League Final soccer match between Arsenal and Chelsea at the Olympic stadium in Baku, Azerbaijan, Wednesday, May 29, 2019. (AP Photo/Luca Bruno) Luca Bruno AP Player ratings from Chelsea vs Arsenal David Luiz - 7 Soccer Football - Europa League Semi Final Second Leg - Chelsea v Eintracht Frankfurt - Stamford Bridge, London, Britain - May 9, 2019 Chelsea's David Luiz celebrates after the match Action Images via Reuters/John Sibley JOHN SIBLEY Action Images via Reuters Player ratings from Chelsea vs Arsenal Andreas Christensen - 7 Arsenal's French striker Alexandre Lacazette (L) and Chelsea's Danish defender Andreas Christensen vie for the ball during the UEFA Europa League final football match between Chelsea FC and Arsenal FC at the Baku Olympic Stadium in Baku, Azerbaijian on May 29, 2019. (Photo by Yuri KADOBNOV / AFP)YURI KADOBNOV/AFP/Getty Images YURI KADOBNOV AFP/Getty Images Player ratings from Chelsea vs Arsenal Emerson - 7 Soccer Football - Europa League Final - Chelsea v Arsenal - Baku Olympic Stadium, Baku, Azerbaijan - May 29, 2019 Chelsea's Emerson Palmieri in action REUTERS/Lee Smith LEE SMITH REUTERS Player ratings from Chelsea vs Arsenal N'Golo Kante - 6 Chelsea's N'Golo Kante, second left, passes the ball to teammate Jorginho, right, as Arsenal's Granit Xhaka, left, and Mesut Ozil watch during the Europa League Final soccer match between Chelsea and Arsenal at the Olympic stadium in Baku, Azerbaijan, Wednesday, May 29, 2019. (AP Photo/Dmitri Lovetsky) Dmitri Lovetsky AP Player ratings from Chelsea vs Arsenal Jorginho - 6 Arsenal's German midfielder Mesut Ozil (front) and Chelsea's Italian midfielder Jorginho vie for the ball during the UEFA Europa League final football match between Chelsea FC and Arsenal FC at the Baku Olympic Stadium in Baku, Azerbaijian on May 29, 2019. (Photo by Yuri KADOBNOV / AFP)YURI KADOBNOV/AFP/Getty Images YURI KADOBNOV AFP/Getty Images Player ratings from Chelsea vs Arsenal Mateo Kovacic - 6 epa07610960 Mesut Oezil (L) of Arsenal in action against Mateo Kovacic (C) of Chelsea during the UEFA Europa League final between Chelsea FC and Arsenal FC at the Olympic Stadium in Baku, Azerbaijan, 29 May 2019. EPA/YURI KOCHETKOV YURI KOCHETKOV EPA Player ratings from Chelsea vs Arsenal Pedro - 7 BAKU, AZERBAIJAN - MAY 29: Nacho Monreal of Arsenal is challenged by Pedro of Chelsea during the UEFA Europa League Final between Chelsea and Arsenal at Baku Olimpiya Stadionu on May 29, 2019 in Baku, Azerbaijan. (Photo by Alex Grimm/Getty Images) Alex Grimm Getty Images Player ratings from Chelsea vs Arsenal Olivier Giroud - 8 BAKU, AZERBAIJAN - MAY 29: Olivier Giroud of Chelsea is challenged by Sokratis Papastathopoulos of Arsenal during the UEFA Europa League Final between Chelsea and Arsenal at Baku Olimpiya Stadionu on May 29, 2019 in Baku, Azerbaijan. (Photo by Dan Mullan/Getty Images) Dan Mullan Getty Images Player ratings from Chelsea vs Arsenal Eden Hazard - 8 Soccer Football - Europa League Final - Chelsea v Arsenal - Baku Olympic Stadium, Baku, Azerbaijan - May 29, 2019 Chelsea's Eden Hazard reacts REUTERS/Lee Smith LEE SMITH REUTERS

However, Uefa are understood to be unhappy over the gates being opened once the game began to let fans in to fill up the stadium, with the Azerbaijan Football Association embarrassed by the small turnout and reportedly taking the action themselves without Uefa’s permission.

“We were in the Arsenal corner and there were a lot of empty seats and suddenly after about 30 minutes lots of local people arrived, including some kids, and they had been given Arsenal flags to wave,” one supporter told The Times.

It’s understood that Uefa are extremely unhappy with the decision being made without their knowledge and are investigation how the turnstiles were opened without their knowledge, leaving them powerless to prevent it.

The European governing body’s concern stems from the fact that Baku is die to host four matches at Euro 2020 next year, and it follows a fierce backlash against the final being staged in the Azerbaijan capital due to the fact that large numbers of supporters were unable to travel there due to extremely expensive travel prices and few direct flights.

Large empty areas of the Olympic Stadium during the Europa League final started to fill up during the match (Reuters)

On top of that, Arsenal midfielder Henrikh Mkhitaryan decided against travelling with the squad due to his safety fears as an Armenian in Azerbaijan, due to the ongoing political conflict between the two sides. Ahead of the game, Arsenal fans wearing Mkhitaryan shirts in Baku were seen being stopped by police.