LONDON — Is Scotland on the way to separating from England, turning the United Kingdom of England, Scotland, Wales and Northern Ireland into something different, maybe a federal kingdom?

The possibility used to be considered remote. After all the Union of Scotland and England has been in place for 300 years — since 1707 — cemented by detailed institutional and political terms. And the two nations shared a common monarch for about 100 years before that. It has been a very profitable union, bringing enormous benefits to both sides.

But now a number of forces have come bubbling and hissing to the surface which are threatening seriously to unravel this marriage, with major implications for both parties.