The Preliminary Declaration in the Constitutions of the United Grand Lodge of England states:
‘By the solemn Act of Union between the two Grand Lodges of Freemasons of England in December 1813 it was declared and pronounced that pure Antient Masonry consists of three degrees and no more, viz., those of the Entered Apprentice, the Fellow Craft, and the Master Mason, including the Supreme Order of the Holy Royal Arch ’.
A detailed explanation can be found by following this link:
www.supremegrandchapter.org.uk/about-royal-arch-masonry/what-is-royal-arch-masonry