Under modern constitutional conventions, the sovereign generally acts on, and in accordance with, the advice of his or her ministers
However, there is some disagreement among scholars as to whether the monarch should withhold royal assent to a bill if advised to do so by her ministers. Since these ministers most often enjoy the support of parliament and obtain the passage of bills, it is improbable that they would advise the sovereign to withhold assent. Hence, in modern practice, the issue has never arisen, and royal assent has not been withheld.
The sovereign is generally believed not to legally have the power to withhold assent from a bill against the advice of ministers.