The game's rule-making body IFAB in December gave the go-ahead for trials of permanent concussion substitutes following which the Premier League and Women's Super League introduced the system in February.
But concerns have been raised after West Ham United defender Issa Diop and Sheffield United defender George Baldock were allowed to continue playing after sustaining head injuries. "Since the beginning of IFAB's permanent concussion substitute trial, we have seen several incidents where the new laws of the game have fallen short of their objective and jeopardised player safety," the letter stated.
"In England... we are aware of two incidents where a temporary option would have better protected players. These cases underline our concern that permanent subs do not give medical teams appropriate environment to assess a player."
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