The highly-rated 19-year-old centre-half, who has been a regular in the Ajax side for two years, is being monitored by a number of European heavyweights.
It is expected Ajax will let De Ligt leave in the summer but, according to Spanish newspaper 'Mundo Deportivo', they won't let him leave cheaply.
The original price for De Ligt was quoted as £44m, but that has now been stepped up by £8m after Ajax became aware of the widespread and high-profile interest in him.
Juventus and Barcelona are leading the chase, but Manchester City and Bayern Munich have also expressed an interest in the Holland international.
De Ligt is a graduate of Ajax's prestigious academy and made his first-team breakthrough in 2016.
He has since played 80 times in all competitions for the Amsterdam club, scoring eight goals and contributing four assists.