England kickstarted their campaign with a 1-0 victory over the Croats thanks to a goal by Raheem Sterling, with 22,500 supporters in attendance at Wembley.
One of those fans was taken to hospital in a “serious condition” after becoming injured in an incident shortly after kick-off.
“We can confirm that a spectator fell from the stands just after kick-off in the match between England and Croatia at Wembley Stadium,” a spokesperson for the venue said.
“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.”
Those two nations go head-to-head on Monday evening.