Witnesses said the fan fell just after kick-off at 2.00pm this afternoon, and the stadium's press team later confirmed that the person was taken to hospital in a serious condition.
"It seemed to take staff ages to get to him. One guy near us who saw the whole thing was crying his eyes out."
Another said: "I didn't see it but spoke to some of the guys who did at half time. They were traumatised. He fell from the balcony towards the seats and the concrete."
A statement from Wembley Stadium reads: "We can confirm that a spectator fell from the stands just after kick off in the match between England and Croatia at Wembley Stadium.
"The spectator was given on-site medical attention and was then taken to hospital in a serious condition.
"We will continue to work with UEFA to ensure the matter is fully investigated and we are continuing to monitor the situation."
LADbible has contacted the FA for comment.
England eventually won the game thanks to a goal from Raheem Sterling - who was born in the area local to the stadium - following good work from Leeds' Kalvin Phillips.
2018 World Cup finalists Croatia were disappointing, but managed to keep England honest until the very end.
England probably took the first half on points, with the only notable chance a whipped effort from Manchester City's Phil Foden.
That shot struck the Croatian post and flashed across the face of goal.
Outside of that, a free kick effort from Mason Mount was probably the most exciting moment of a match that perhaps suffered as a result of the oppressive heat in London.
That said, England and their manager Gareth Southgate won't mind, given that this narrow victory is the first ever occasion that the Three Lions have won their first fixture at a European Championship.
At the end of the match, cheers from the 25,000 fans inside Wembley rang out, as much in relief as in triumph.
England will now turn their attentions to a huge match-up against Scotland back at Wembley on Friday evening.
Win that, and their progression to the round of 16 will be all but guaranteed.
Featured Image Credit: PA