Former Prime Minister Sir John Major has said an independent Scotland would be in a "much weaker position to influence" its interests in the world.
He told BBC Radio 4's Today programme that he believes some Scots hare not aware of the full implications of independence, which he accused the SNP of "glossing over".
He said there should be no "assumption that Scotland will be waved into the European Union," and that even if it was, it would find it harder to fight for its own interests.
"Why would Nato let them in," he asked, when the SNP plans to scrap the Trident nuclear deterrent.
It would be a "truly dreadful outcome" for both the UK and the EU if the result of the referendum - promised by Mr Cameron if the Tories win the next general election in 2015 - is that Britain decides to leave, Sir John Major said.
Of course, we would survive, but there would be a severe price to pay in economic well-being, in jobs and in international prestige.
In a world of seven billion people, our island would be moving further apart from our closest and largest trading partners, at the very time when they, themselves, are drawing closer together. This makes no sense at all.
Britain will pay a "severe price" if it votes to leave the European Union, Sir John Major warned.
The former prime minister argued that the exit could cost billions, and would leave the UK isolated internationally yet still required to implement EU regulations it had no part in framing.
Sir John - whose premiership was scarred by long-running battles with Tory Euro-sceptics - backed David Cameron's strategy of renegotiating Britain's membership before staging an in/out referendum as the best way of finally resolving the Europe issue.
Social mobility is being held back in part by a lack of aspiration from people who are not white and middle class, David Cameron suggested.
Endorsing Sir John Major's call for action counter the "truly shocking" dominance of privately educated affluent individuals in powerful positions, he insisted it was not enough just to "make changes and sit back. He said:
"You've got to get out there and find people, win them over, get them to raise aspirations, get them to think they can get all the way to the top."
Former Prime Minister Sir John Major has criticised the number of privately educated and wealthy middle-class in the “upper echelons” of power in the UK, which he dubbed "truly shocking".
In a speech at South Norfolk Conservative Association on Friday, Major gave a stinging assessment of social mobility in 21st century Britain, with remarks that will hurt Conservative leader and Old Etonian David Cameron.
"In every single sphere of British influence, the upper echelons of power in 2013 are held overwhelmingly by the privately-educated or the affluent middle class.
"To me, from my background, I find that truly shocking."
However, the former Conservative Prime Minister levelled the blame for the collapse in social mobility with previous Labour governments, who left a "Victorian divide between stagnation and aspiration".
Iain Duncan Smith has taken a swipe at his former Tory rival John Major today over the former prime minister's traffic cones hotline initiative during the 1990s.
The Secretary of State for Work and Pensions hit back after Major this week questioned his former colleague's ability to reform benefits.
Duncan Smith told the Evening Standard: “Well, as I say, I never really get too fussed about what people think about their own intellects. I’m always happy to be in awe of someone whose own intellect delivered us the cones hotline, I must say.”
Major's infamous telephone hotline was set up for people to call to complain about unattended roadworks and has gone down in political history as one of the most ridiculous initiatives launched by the British Government.
At the moment I do not see how it can be in any way acceptable that with energy prices rising broadly 4% in terms of costs that the price to the consumer should rise by the 9-10% that we are hearing.
I do not regard that as acceptable at all by the energy companies.
And it is not acceptable to me, it ought not to be acceptable to anyone, that many people are going to have to chose between keeping warm and eating. That is not acceptable.
So if we get this cold spell the government, I think, will have to intervene and if they do intervene, and it is costly, I for one would regard it as perfectly acceptable for them then, subsequently, to levy and excess profits tax on the energy companies and claw that money back to the Exchequer, where their primary job is to get the economy working and people back to work.
Downing Street has said the Government had "no plans" for a windfall tax of the type proposed by Sir John Major.
David Cameron's official spokesman told a regular Westminster media briefing: "The Prime Minister's view on this is that this is an interesting contribution. We have no plans for this."
"What the Government is doing is legislating around forcing energy companies to put customers on their lowest tariffs and more competition in energy markets."
Asked about Sir John's concerns about people having to choose between eating and heating this winter, he said:
"There are a number of initiatives that the Government has to support vulnerable people, such as the cold weather payments.
"We have a range of ways in which support is given and those are the right ones."
Labour leader Ed Miliband said former Prime Minister Sir John Major was making "Labour's argument" by calling for energy companies to face an excess profits tax.
Sir John Major makes Labour’s argument: David Cameron stands up for the energy companies not hard-pressed families.