Answered by Shaykh Amjad Rasheed
Can in a state of major ritual impurity who wants to fast remain in that state until dawn without ghusl?
Yes, it is permissible for him to do that and his fast is still valid. It has been affirmed that he, peace and blessings be upon him, used to be in major ritual impurity in the morning, and he was fasting, and he would take his ghusl to pray the fajr prayer. This was done by him, peace and blessings be upon him, to show its permissibility, though it is best for the junub to perform his ghusl before dawn.
Amjad Rasheed
[Translated by Sr. Shazia Ahmad]