Tony Blair yesterday delivered his most impassioned and personal defence of the Union between Scotland and England, insisting both benefited economically from the constitutional marriage and divorce would be "crazy".

Speaking at his monthly press conference on the 300th anniversary of the signing of the Act of Union, Mr Blair insisted it should be "celebrated with pride" and independence would result in "catastrophic consequences for the Scottish economy".

Declaring that Scotland benefited in terms of jobs and security, the Prime Minister said: "It would be crazy for Scotland to be wrenched out of the United Kingdom."

He sought to emphasise the family links between the nations, pointing out how, in 1707, there were about 30,000 Scots, 3% of Scotland's population, with relatives in England. Now there are 2.5 million or half the population.

The Edinburgh-born and educated premier used his own background to underline how the peoples of the British Isles shared a common bond. "My mother was born in Ireland, my father was born in England but brought up in Scotland, I was born in Scotland but brought up in England. The idea that in today's world when everyone is moving closer together you separate Scotland out from England and end up with a Scottish separate currency, separate interest rates, it would just be disastrous."

Mr Blair dismissed suggestions he was using the politics of fear to scare Scots into backing the Union: "I make the case on the basis of the benefit the Union has been to Scottish people and to English people. Devolution provides a sensible way forward and separation is just a retreat into an old-fashioned view of the world."

Asked about why there were no glitzy events to mark the 300th anniversary - last night, Douglas Alexander, the Scottish Secretary, launched a special £2 coin at a Whitehall reception - Mr Blair replied: "The most important thing is not fireworks but argument and giving a coherent reason as to why the Union of England and Scotland is good for today's world and the future."