Major told BBC radio that a vote to leave the EU would mean a "high probability" that Scotland would have a referendum of its own and decide to leave the UK Scotland voted last year to remain part of the UK after a bruising referendum campaign. Its leaders want to remain in the EU.
"The United Kingdom would be fractured, that would be very damaging," he said today.
British Prime Minister David Cameron is seeking concessions from Europe's leaders at a summit this week ahead of a planned referendum on Britain's membership to be held by the end of 2017.
But the German leader told lawmakers Wednesday that the issues of freedom of movement and equal treatment of EU citizens are non-negotiable.
Merkel said "these principles are not up for discussion" but that Germany wants the UK to "remain an active partner in a strong European Union.