| Gallo-Brittonic | |
| Also Known As: | P-Celtic |
| Region: | Gaul and Great Britain |
| Familycolor: | Indo-European |
| Fam2: | Celtic |
| Child1: | Gaulish |
| Child2: | Brittonic |
| Glotto: | none |
The Gallo-Brittonic languages, also known as the P-Celtic languages, are a proposed subdivision of the Celtic languages containing the languages of Ancient Gaul (both celtica and belgica) and Celtic Britain, which share certain features. Besides common linguistic innovations, speakers of these languages shared cultural features and history. The cultural aspects are commonality of art styles and worship of similar gods. Coinage just prior to the British Roman period was also similar. In Julius Caesar's time, the Atrebates held land on both sides of the English Channel.
It contrasts with the Insular Celtic hypothesis, which asserts that Goidelic and Brythonic underwent a period of common development and have shared innovations to the exclusion of Gaulish, while the shared changes are either independent innovations that occurred separately in Brythonic and Gaulish or are due to language contact between the two groups.
The hypothesis that the languages spoken in Gaul and Great Britain (Gaulish and the Brittonic languages) descended from a common ancestor, separate from the Celtic languages of Ireland, Spain, and Italy, is based on a number of linguistic innovations, principally the evolution of Proto-Celtic *kʷ into pronounced as //p// (thus the name "P-Celtic"). These innovations are not shared with the Goidelic languages.
The proposed shared innovations not in Goidelic are: