The warning by the military alliance was issued as its 28 members agreed to a minimum defence budget of 2 per cent of GDP at its summit in Newport, Wales.
The Scottish Government has said that defence spending would stand at £2.5 billion, or 1.7 per cent of GDP, after a Yes vote.
In response, the SNP claimed “Westminster’s own defence spending is due to fall below the 2 per cent figure”, and described Conservative criticism of its plans as “patently absurd”.
The decision to up spending by Nato members came as Prime Minister David Cameron said the events of this week in Iraq and Ukraine spoke of “a very dangerous and insecure world”, and claimed this underlined why Scotland must remain in the UK.
The UK government also said a pledge by Mr Cameron yesterday to bring the Royal Navy’s second new aircraft carrier The Prince of Wales into full use – instead of being mothballed as planned – will protect defence jobs at Rosyth in Fife.
The “Wales Pledge” on spending, signed at the Nato summit, is intended to strengthen the commitment of member states on defence spending at a time when the West is facing threats from the Islamic State and a stand-off with Russia.
At present, Britain, the United States, Estonia and Greece are the only countries to meet the new target but the other members agreed to increase their spending to bring it to 2 per cent.
However, the Scottish Conservatives said the Scottish Government’s own figures showed that it proposes to spend £500m “too little” on defence if voters back independence.
The SNP plans laid out in its white paper show a commitment to spend £2.5bn per year. The Scottish Government has said Scotland would have a GDP of £148bn, meaning it would spend 1.7 per cent of GDP.
The white paper also specifically states that the Scottish Government would “reduce defence and security spending to £2.5bn per year” which would release more than half a billion pounds to help pay for pledges like maintaining free personal care, funding triple-lock pension plans, reducing energy bills and paying for free childcare.
Asked how this would affect an independent Scotland’s application, a Nato spokesman told The Scotsman: “All member states have signed up to the 2 per cent pledge; therefore any theoretical new member state would have to abide with that.”
Earlier, a senior Nato source also warned that a separate Scotland could not “simply walk into” the organisation.
He said: “Joining Nato isn’t just a question of applying and walking in. The bottom line is that you need every single Nato member to say Yes for you to join and even if one doesn’t, you’re stuck.”
At a news conference at the summit, Mr Cameron was asked about how independence would affect his pledges on defence made this week.
In response, he accused Alex Salmond of promoting “total confusion” on whether or not an independent Scotland would join Nato, the Europe Union or what currency it would use.
“It is for him to answer questions about what would happen,” Mr Cameron said.
He said in the last two weeks of the campaign, he would be taking to Scotland a “very loud and clear message, which is: We want you to stay. We care passionately about our family of nations”.
He went on: “I don’t think anyone can be in any doubt that we live in a very dangerous and insecure world and I would have thought one of the strongest arguments that those of us who want to see the UK stay together can make is, in that dangerous and insecure world, with terrorist and other threats, isn’t it better to be part of the United Kingdom, that has a top-five defence budget?
“That has some of the best security and intelligence services anywhere in the world, that is part of every single alliance that really matters in the world in terms of Nato, the G8, the G20, the EU, a permanent member of the security council of the UN.
“To have all those networks and abilities to work with allies to keep us safe, isn’t it better to have those things than separate yourself from them?”
Reacting to the claimed shortfall in defence spending Scotland would face, Scottish Conservative leader Ruth Davidson said: “The SNP’s stated aim is to disarm a nuclear member of Nato – despite the rules of entry requiring every member to sign up to Nato’s first-strike nuclear policy. Now, we see an independent Scotland’s military budget would be £500m short.
“This is yet more proof that Alex Salmond is utterly clueless when it comes to defence.”
However, the SNP dismissed the attack on its plans as “a boomerang attack by the Tories”.
SNP defence spokesman Angus Robertson said: “This is a boomerang attack for two reasons. First, Westminster’s own defence spending is due to fall below the 2 per cent figure, so on their logic the UK will be expelled from Nato, which is patently absurd.
“The UK’s own recently retired ambassador to Nato, Dame Mariot Leslie, has also said that an independent, non-nuclear Scotland will be welcomed as a member of the alliance.”
He went on: ““The Scottish Government is committed to the European Nato average on defence spending, some 1.7 per cent of GDP – and the SNP is, of course, committed to getting rid of Trident nuclear weapons and increasing conventional defence such as maritime patrol aircraft which the UK government scrapped.”
He added: “It was Westminster’s decision to wage a disastrous and illegal war in Iraq – backed to the hilt by the Tories as well as Labour in 2003 – which has made the world a much more dangerous place, the effects of which are still with us.”