A regius professor is literally a king’s professor and holds a chair that, historically, was endowed by a monarch. Regius professorships are generally considered the highest status of a professorship in the UK. When they were first founded in the 15th and 16th centuries, regius professorships were endowed by the monarch, which meant that the crown paid the professors’ stipends. Regius professorships are now created by the government, not the monarch, and it is the university holding a regius professorship that must cough up the incumbent’s salary and any associated costs.
A Regius Professorship is a rare privilege, with only two created in the past century in UK. It is a reflection of the exceptionally high quality of teaching and research at an institution
SOURCE: https://professors.leeds.ac.uk/what-is-a-professor/regius-professorships/