A vote for Scottish independence would be "disastrous" for the whole of the United Kingdom, former prime minister Sir John Major has warned.

In an impassioned plea to Scots not to break up "the most successful union" in history, Sir John said they would face an uncertain future alone while the rest of the UK would be diminished on the world stage.

"I am desperately concerned at what is happening. We would be immensely weaker as a nation in every respect - morally, politically, in every material aspect - if Scotland and the rest of the United Kingdom were to part company," he told the BBC Radio 4 Today programme.

"This year is the 100th anniversary of the First World War. As we honour the people who fought together then, would it not be extraordinary if the SNP broke up the most successful union and partnership in all history in any part of the world?"

Sir John said that young Scots considering casting a vote for independence in next week's referendum would find themselves living in "a very different country" over the coming decades if there was a Yes vote.

"There is easy assumption that Scotland will be waved into the European Union. Well, maybe after a period of years that's possible but it's not certain. Spain for example wouldn't welcome a separatist nation," he said.

"There is the belief they are going to be able to get into Nato. Why would Nato let them in when the SNP propose to wreck the Trident independent nuclear weapon?

"If Britain lost Trident it would have lost what has shielded it and protected it for a long time - including Scotland. For that to be done by the enemy would be a disaster, to have it done by friends is almost unbelievable."

Sir John said that the lose of the Trident nuclear deterrent - which is currently based in Scotland - would have consequences for the whole of the UK.

"Our role in Nato would be reduced, our relations consequently with the United States would be damaged," he said

"The United Kingdom would be weaker in every international body it attends. We would lose our seat at the top table in the United Nations."

The former premier - who warned 20 years ago that Scottish devolution would be "the high road to separation" - was scathing about the SNP's inability to say what currency they would use if they did achieve independence.

"It is one week away from the vote and people of Scotland do not know what currency they will have - I've never known such incompetence," he said.

Asked if David Cameron regarded Sir John's intervention in the referendum debate as helpful, the Prime Minister's official spokesman told a regular Westminster media briefing: "He very much welcomes the voices we are hearing - including from Sir John - making the case for the UK staying together.

"At the heart of the Prime Minister's message - and I think it came through clearly in Sir John's - is that we want the people of Scotland to stay part of the UK."

Asked whether Mr Cameron shared Sir John's assessment that the UK would lose its permanent seat on the UN Security Council if Scotland left the union, the spokesman said: "The Prime Minister's assessment starts from the basis that Scotland and the UK are stronger together."