The Prime Minister said the United Kingdom had the best security and intelligence services in the world to keep people safe.
He said the safety of staying together in a ‘very dangerous and insecure world’ was one of the ‘strongest arguments’ against separation.
Mr Cameron, speaking at the end of the two-day Nato conference in Wales, said: ‘I don’t think anyone can be in any doubt we live in a very dangerous and insecure world.
‘I would have thought one of the strongest arguments that those of us who want to see the United Kingdom stay together can make is in that dangerous world and insecure world of terrorist threats and other threats isn’t it better to be part of the United Kingdom?’
He said the UK ‘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 European Union, a permanent member of the Security Council of the UN’.
Mr Cameron added: ‘To have all those networks and abilities to work with allies to keep us safe, isn’t it better to have those things than to separate yourselves from them.
‘Those are some of the arguments we will be making in the weeks to come before this vital vote.’
Mr Cameron's remarks came after the former Prime Minister Gordon Brown vowed to lead a campaign for more powers for Scotland if it rejects independence in this month’s referendum.
The former prime minister, signalling his return to front line politics, said he would push for further devolution within weeks of the September 18 vote.
Mr Brown is among senior Labour figures being deployed in a final push by the party to prevent its supporters being won over by the Scottish National Party.
With polls suggesting a late swing towards the Yes camp, Mr Brown urged voters not to ‘abandon’ the huge value to Scotland of pooling resources with the rest of the UK in areas such as pensions and healthcare.
He told an audience of activists and politicians at Westminster that he had asked Speaker John Bercow to allow him to lead a debate when the Commons resumes business in October to galvanise cross-party support for reforms.
A pledge of extra tax and legal powers for Holyrood in the event of a rejection of independence in the popular vote has been signed by the leaders of all three main Westminster parties.
‘The status quo is not now an option. A no vote does not mean no change.’
He said: ‘A no vote will usher in the further constitutional reforms in social, economic and fiscal affairs and a no vote will mean the choice you have made is against separation but for further devolution to the Scottish Parliament.
‘Over the next few months after the referendum, we can take the lead in ensuring that these powers - that the Conservatives and Liberals say they would support - can be implemented as quickly as possible.
‘I have asked the Speaker that when Parliament returns after the referendum in October, that in the first week he allows me to lead a debate about how we can secure the timetable and the delivery of the further devolution proposals.
‘This is not like 1979 when the then-Conservative government promised change and then denied us it. This is like 1997 when the Labour government came in and, within a year, had delivered its promise of a Scottish parliament.
‘When we are talking about the future of the constitution, London must and will change, Westminster must and will change, and, of course, the United Kingdom must and will change.’
He told the meeting - hosted by Blairite pressure group Progress - that the No camp ‘will accept whatever result comes our way’.
‘But the Scottish National Party have got to recognise that if they lose this ... we have got to get back to the issues of improving public services.
‘I think the Conservatives will have to accept these proposals ... the strong weight of opinion for them.
‘We stand ready to talk to other parties about how we could reach a consensus on these proposals for the future.’
Mr Brown said the UK was a uniquely successful arrangement anywhere in the world.
It offered pooled resources across four nations ‘to guarantee to anyone no matter where they are in any part of the United Kingdom that they have the same right to healthcare, to help when unemployed, to dignity in retirement and, of course, to dignity and support when they are sick, disabled or vulnerable.
‘That is a principle that we should never abandon.’
He dismissed SNP plans for a corporation tax cut as ‘the Scots Nats backing the fat cats and obviously cheered on by the cybernats’.
He also dismissed the level of demand for an English parliament.
‘The English do not want a parliament of their own. If they wanted it, that would be an issue, but there is no evidence that there is a desire for an English parliament,’ he said.
‘We have got to recognise the assymetric nature of the British constitution, find a way of accommodating the ambitions of all the different nations - and indeed the regions.’
