In coverage of the ongoing Brexit votes, I'm seeing individuals being referred to as “arch-remainers”. Google also suggests there's some minor usage of the term “arch-leaver”. I've never heard this terminology before, possibly because I haven't been closely following UK politics until recently.
This is from the Labour peer Andrew Adonis, who is an arch-remainer.
What does arch-remainer here mean? Does that mean they're just a big figure in the remainer scene?