Ola Billger, 40, used binoculars to spy on England coach Roy Hodgson as he outlined his defensive plan to his team at a Kiev hotel ahead of last Friday's Group D clash.
The Swedish hack watched for 40 minutes and compiled detailed notes as Hodgson ran his players through his strategy for defending free-kicks and corners.
Billger then passed on the information to his own national team, which gladly accepted the dossier.
England went on to concede twice from free-kicks to Sweden before coming back to win 3-2, knocking the Scandinavians out of the competition.
Billger, who writes for Stockholm paper Svenska Dagbladet, said he stumbled on the briefing by chance, but was not going to let the opportunity to grab an advantage for his nation slip away.
"I just happened to look out of my third-floor window when I saw Roy Hodgson through a glass roof below," he told The Sun. "I realized I was watching his team briefing and slideshow so decided to take a closer look with my binoculars."
He added, "England decided to lay all their secrets out before me. What was I supposed to do -- look away? I called the Swedish camp straight away and they seemed delighted."
An England source told The Sun, "It's not nice being spied on and bad form of the Swedes to accept information obtained in such an underhand fashion."