What is the ruling on one who delays making up missed prayers without a legitimate Shar'i excuse until the time for the next prayer begins, and is that considered an act of disbelief (kufr)?

Whoever misses a fard prayer should immediately make it up. Some jurists hold that making it up is obligatory immediately, while others believe it can be done at leisure. Delaying the making up of a prayer is not considered disbelief (kufr); rather, it is a transgression against what is obligatory and an act of disobedience according to the view that immediacy is واجب (obligatory).