Earlier this month the ex-Leeds and Juventus star underwent emergency vascular surgery after collapsing prior to an appearance on Italian TV.
However, problems with circulation have now led to the need for further surgery.
Awarded the CBE in 2001, he won 38 caps for Wales and scored 15 goals.
He played in the last Wales team to qualify for the World Cup, when he starred at the 1958 finals.
72-year-old Charles remains popular at Juventus and the news of his illness has made headlines in Italy.