Footballers who regularly head a ball are risking a “glaring” decline in memory and eye speed, according to researchers in Liverpool.
Liverpool Hope University tested amateur players and noticed adverse effects after only a short period of heading the ball.
It comes as former Tottenham, Hull and England midfielder Ryan Mason called for a ban on children heading footballs to be introduced.
The 27-year-old was forced to retire in February 2018 after suffering a fractured skull during an aerial challenge in a Premier League game against Chelsea 13 months earlier.
A ban would copy rules in the United States, where under-11s are outlawed from heading by the the US Soccer Federation - with limitations in place for players between 11 and 13.
"If you have got a seven or eight-year-old heading a solid ball, and his brain and his bone in his skull isn't fully developed, then that could potentially be doing damage," Mason told the BBC
"I look at some kids and they head the ball with the top of the head and their technique is all wrong, therefore the pressure that it's putting on the brain is a lot more.
"I don't think kids should be heading real balls."
The Professional Footballers' Association (PFA) has previously called for restrictions for youngsters to be introduced in the UK until the long-term health risks of heading are better understood.
Dr James Roberts and Jake Ashton, who carried out the tests at Liverpool Hope, said: “In our study there was a short bout of 20 headers with the player having the ball thrown to them and heading it back
“When tested immediately afterwards it showed there were clear effects – it was quite glaring.
“There was a decline in both saccadic eye speed and short-term memory which is concerning when you consider most recreational players will have to go about their everyday lives such as drive home after a game.”
Dr Roberts said further testing including a control group would be required before any definitive conclusions can be made.