AN independent Scotland could face being blocked from Nato membership if it failed to stump up an extra £500 million to meet the alliance's new pledge on spending, it has been suggested.

The warning came as David Cameron insisted that in a dangerous and insecure world Scotland would be much safer within the UK with all its diplomatic, intelligence and military networks than if it became independent.

At present, just four of the 28 Nato members, including the UK and the US, spend more than two per cent of GDP on defence but an agreement at the summit will mean others will seek to increase theirs to that level.

The Scottish Conservatives calculated, using the SNP Government's own figures, that Scotland's current GDP was £148bn but given it pledged to spend £2.5bn a year on defence should there be a Yes vote, there would be a shortfall of £500m in terms of meeting the Nato level.

Ruth Davidson, the party leader, insisted the Nationalist claim that an independent Scotland would "waltz into Nato" was simply untrue and said the £500m shortfall proved Mr Salmond was "clueless" on defence.

"Independence would damage our armed forces and hurt our security," declared Ms Davidson.

At the summit, a Nato spokesman confirmed: "All member states have signed up to the two per cent pledge, therefore any theoretical new member state would have to abide with that."

A senior diplomatic source also made clear an independent Scotland's path towards membership might not be straightforward, saying: "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."

In October 2012, the SNP ditched its anti-Nato policy after the leadership recommended the change, believing it was important to underpin the party's defence credentials. Crucially however, the switch is dependent on Scotland getting rid of nuclear weapons, which the First Minister has promised to do by 2021 in the event of a Yes vote.

Angus Robertson MP, SNP defence spokesman, said: "This is a boomerang attack by the Tories. Westminster's own defence spending is due to fall below the two 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 already said that an independent, non-nuclear Scotland will be welcomed as a member of the alliance.