“The picture of the linguistic landscape of Britain and Ireland will be of interest not only to linguists but to historians, archaeologists and archaeogeneticists.”

Dr Simon Rodway, of Aberystwyth University, is part of the team bringing together disparate sources to compile the dictionary. Photograph: Aberystwyth University

Elements of modern languages such as Welsh, Irish, Scottish Gaelic, Breton and Cornish have some roots in their ancient Celtic counterparts.

The team compiling the dictionary say that while modern Celtic languages are often different from each other, similarities can be seen between words.

For example, the words for sea in Welsh and Old Irish – môr and muir – correspond to “Mori” in Celtic names such as Moridunum, which means “sea fort” and is an ancient name for Carmarthen in south-west Wales.