There is ongoing dispute around its origins.

British sources have a belief and can't objectively prove it was first made there:

“One story purports that it was invented in the 1970s by Ali Ahmed Aslam, a Pakistani‑Scottish chef in Glasgow, who, to please a customer, added a mild tomato‑cream sauce to his chicken tikka" - Brittanica

“Ali Ahmed Aslam, a Pakistani‑immigrant chef in Glasgow, claimed he invented chicken tikka masala in the early 1970s using canned tomato soup and spices.” - NPR

Punjabi sources claim it had already been done at least a decade prior, also can't be proven.

“A recipe for ‘Shahi Chicken Masala’ appears in this 1961 Indian cookbook, predating the Glasgow claim by a decade.” - Balbir Singh’s Indian Cookery (1961)

“My grandfather was serving chicken tikka masala to Indian heads of state as early as 1947.” - as reported by NPR from an interview with Monish Gurjal chairman of Moti Mahal (one of the first restaurants to introduce Punjabi and North Indian cuisine to the rest of the world)