"I couldn't believe my ears or my eyes," said Middlesbrough's Swedish coach Kim Hellberg at a press conference after the match.
A person from Southampton is suspected of having illegally filmed Middlesbrough's training ahead of Saturday's Premier League qualifying match.
The league's regulations prohibit clubs from "observing, or attempting to observe, another club's training session within 72 hours before a scheduled match between two clubs."
"I just think it's strange that they do that and try to cheat in these types of matches. It's just wrong," Hellberg said.
The winner of the doubles match will face Hull or Millwall in the qualifying final.
