Under clause 15 of Paragraph 204 Territorial Army Regulations.

A man of the Territorial Army may enter the Royal Navy or enlist into the Regular Army, the Militia (Supplementary Reserve, Categories A and B only), the Royal Marines or the Royal Air Force, and, on final approval, is deemed to be discharged from the Territorial Army, but must return in good order the articles of clothing, &c., issued to him belonging to the Territorial Army. He is not, however, allowed to join the Royal Naval Reserve or the Army Reserve (other than the Supplementary Reserve, Categories A or B.)