The Belgian Football Association has been urged to take swift action after Standard Liege player Uche Agbo was racially abused at a game.
Agbo, at Standard on loan from Watford, was sent off for a second booking in a defeat at Courtrai on Wednesday, contesting the decision and taking more than three minutes to leave the pitch.
As he walked back to the dressing room, he was subjected to monkey chants and pelted with plastic cups and responded with an obscene gesture.
The authorities have now been urged to take action to stamp out the discrimination marring the game.
"It's not the first time I have to ensure that we get clarity in terms of discriminatory words and attitudes," Pierre Francois, the CEO of the Pro League, said.
"There was recently the controversy over the association of Anderlecht fans with Jews by Bruges supporters. And in the same way, it's inadmissible to hear 'Walloons are s---.'
"It's high time to react, something the Belgian FA has not done over a number of years and with the consequences we now know today."
The Belgian FA was criticised for claiming that offensive chants aimed at the French-speaking Walloon part of the country were "mickey-taking and fun."
And Francois called on the organisation to do more to fight racism, adding: "Monkey chants are absolutely inadmissible. That would lead to heavy fines at the UEFA level.
"It's time to react. We have to look after the standard of our football, which isn't always up to scratch, but also ensure the atmosphere in stadiums is no longer toxic."
Standard released a statement which said they "strongly condemn the attitude of those individuals whose intolerable behaviour is totally contrary to the values defended by our club and by the Jupiler Pro League."
Agbo's agent, Miguel Alfaro, said the 22-year-old "doesn't understand how, in this day and age, people can still have that kind of attitude in a stadium."
He added: "But he doesn't want to get into a debate. What he wants is to forget this episode as quickly as possible and focus on the rest of the season."
Abgo was also backed by Watford teammate Christian Kabasele, who was racially abuse at Courtrai while playing for Genk in 2015.
"It's time to take firm and serious measures as is the case for European competitions so that these muppets stay at home for every weekend of football," Kabasele wrote on Twitter.