There is no contradiction in backing nuclear-armed military alliance Nato and voting to scrap the renewal of the UK's nuclear fleet, according to SNP leader Nicola Sturgeon.

The SNP has pledged to vote against the planned renewal of Trident, and polls suggest the party could hold significant sway in the next parliament.

Ms Sturgeon said everyone should be worried about existing nuclear armed states, but said renewing Trident could encourage more countries to arm themselves.

Her party reversed its historic opposition to Nato, which has an open policy on using nuclear weapons first in a conflict, ahead of the independence referendum in a tight vote that led to the resignation of three MSPs.

Speaking on Radio 5 Live Breakfast, Ms Sturgeon said: "On the issue of Nato, during the referendum we said if Scotland was an independent country we would seek membership of Nato.

"I don't agree that that is inconsistent with taking a non-nuclear position.

"Nato has got 28 member states at the moment and 25 of them don't have nuclear weapons, so it's actually normal within Nato for countries not to possess nuclear weapons."

She added: "I am, in principle, opposed to nuclear weapons.

"I think national security is hugely important, of paramount importance, but of the something like 200 countries in the world 190 or so of them don't have nuclear weapons.

"Are we saying that these countries are less safe because of that?

"I think we should all be worried about the ones that do.

"But my argument is if we're trying to encourage other countries either to get rid of their nuclear weapons or not develop nuclear weapons, then I think the last thing we should be doing is spending £100 billion renewing our nuclear weapons because that sends entirely the wrong message to countries that we are seeking to say to them: 'Do not develop your nuclear capacity'.

"Which brings me to the decision that will be taken over the life of the next parliament, which is whether to renew the Trident system and I just believe it is the wrong use of £100 billion."

There were 34 arrests at an anti-nuclear demonstration yesterday at Faslane naval base, which houses the UK's Trident fleet on the west coast of Scotland.

Some protesters had to be cut free from gates as the Scrap Trident Coalition staged a blockade.