Portugal football legend Eusebio was taken to hospital here on Saturday evening after falling ill at the team hotel, the country's football federation said on its website.
It is the fourth time this year that the 70-year-old 1965 Ballon d'Or winner has been hospitalised.
"The ambassador of the national team, Eusebio da Silva Ferreira, fell ill at the end of the day on Saturday at the hotel where the squad are staying," a statement read.
"After consultation with his doctor in Lisbon, he was taken as a precaution to a hospital in Poznan accompanied by a team doctor for further tests."
The Portuguese federation said Eusebio - who attended the 1-0 quarter-final victory over Czech Republic on last Thursday - spent a ‘comfortable’ night in the Miejski hospital, and one of the organisers chiefs Tomasz Stube said his life was not in danger.
"Eusebio is surrounded by good medical care," Stube told Polsih agency PAP.
"We cannot go into the details of his state of health."
The Portugal squad doctor, Henrique Jones, explained that the former Benfica player had had a number of tests, including an electrocardiogram, which revealed ‘some small variations’.
Eusebio was set to have further blood tests on Sunday morning, Jones added.
The player, who was taken to hospital last February in Lisbon for the third time in a little over two months, was being treated for high blood pressure. He has previously had neck pain and pneumonia.
The Mozambique-born former Portugal forward made his name with Benfica, with whom he won a string of domestic and international titles that brought him to foreign attention.
‘The Black Panther’ was also in the national side that finished third in the 1966 World Cup, where he also top scored with nine goals.
Apart from winning the Ballon d'or he was twice the leading European goalscorer.