A doctor is urging armchair football fans to dial 999 if they feel ill watching the World Cup on TV '“ saying spectators experience the same mental exertion as players.
Dr Jeff Foster says fans with weak hearts are particularly at risk if they become too excited while cheering on their team.
He said: "You wouldn't ask someone who'd just had a heart attack to go out for a run but watching a match can be just as bad.
"Your heart rate can go from 60 beats per minute at rest to something like 120/130 bpm during a match and that is the last thing you want.
"I remember at a previous World Cup, one patient of mine had suffered a heart attack while watching an England match but put off calling 999 until the game had finished.
'He was very sick indeed by that time but said he couldn't tear himself away from the game.
'It was probably the adrenalin rush which got him through to the final whistle but he took a huge risk and clearly it's not a gamble worth taking.'
Dr Foster, of TFJ Private GP Services, in Leamington Spa, Warks, said tests showed fans and players experience the same range of emotions both on the pitch and at home.
He added: "Football fans have such a close, sub-conscious connection with their team that it produces a powerful chemical reaction in their brain and body.
"Hormones such as adrenalin and endorphins are released and you have a huge energy surge even though you are maybe just sitting at home watching on TV.
"Incredibly, this surge is so big it is very similar to that experienced by the players who are actually taking part in the match.
The World Cup starts on Thursday as the hosts play Saudi Arabia. (Photo: Shutterstock)
"The adrenalin heightens awareness, it makes all your senses more active and highly vigilant as you develop a real focus, a tunnel vision on what's about to happen.
"It's also what causes palpitations, sweaty palms, anxiety and drives your heart rate up and down.
"Meanwhile, the endorphins, especially seratonin, make you feel really good, really positive and this is reinforced if things go well and so the feel-good factor can build up throughout the tournament if the team is doing well.
"When things don't go well, the body stops pumping the endorphins and the lack of serotonin has the opposite effect."
The World Cup finals kick off in Russia on Thursday (14/6), with England's first group match against Tunisia next Monday (18/6).