Netherlands players heard monkey chants from the crowd at a Euro 2012 training session at Wisla Krakow's stadium this week, a team spokeswoman said today, casting a shadow over what had been a near carnival atmosphere.
The build-up to the tournament being co-hosted by Poland and Ukraine was dominated by worries about racism among home fans.
Coach Bert van Marwijk, Dutch officials and many of the press did not hear any racist abuse during yesterday's session but it has since emerged that a handful of the squad did.
"Some players did hear some monkey noises. That is why they moved to the other side of the pitch," the spokeswoman said.
The Dutch FA (KNVB) did not lodge a formal complaint with UEFA but was contacted by football's European governing body after reports of the incident appeared in local Dutch media.
"UEFA has now been made aware that there were some isolated incidents of racist chanting that occurred at the open training session of the Dutch team in Krakow," UEFA said in a statement today.
"Should such behaviour happen at further training sessions UEFA would evaluate the operational measures to be taken to protect the players.
"UEFA has a zero-tolerance policy when it comes to discriminatory behaviour and has given the power to referees to stop matches in case of any repeated racist behaviour."
UEFA has a film crew at all open training sessions but they did not pick up any racist chanting on their video.
Immediately after Wednesday's session, Van Marwijk had told reporters the atmosphere was fantastic with over 30,000 people turning up just to watch training.
Krakow is not staging any matches.
Netherlands captain Mark van Bommel told Dutch reporters today: "Open your ears. If you did hear it, and don't want to hear it, that is even worse."
UEFA president Michel Platini, under pressure after boldly handing the tournament to the two countries, said players should let the referee handle racist abuse after some players indicated they would walk off the pitch if it occurred in matches.
Italy's Mario Balotelli said last month he would "kill" anyone who threw a banana at him in the street during the European Championship.
Referring to racist chants, he added: "If it happened I would walk off the pitch and return home. We are in 2012. It's not possible."
The Polish and Ukrainian governments have worked hard to try to dispel any fears of racism but a BBC documentary suggested it was prevalent among certain fan groups.
World Cup runners-up Netherlands, who have a number of black players, open their Euro 2012 Group B campaign against Denmark in Ukraine's Kharkiv on Sunday (NZ time)
Yesterday's incident followed a visit by the Dutch to the Nazi death camp at Auschwitz, close to Krakow.
The tournament starts later on Saturday (NZ time) when Poland play Greece in a Group A match Warsaw.
Who was the best player at the World Cup?