Prime minister promises to fight for the United Kingdom and warns Scotland what it could lose by going it alone

David Cameron claimed on Wednesday night that an independent Scotland would be less influential on the world stage, lose the protection of the UK's armed forces and could lose the pound.

Speaking before a trip to Edinburgh, the prime minister talked in passionate language about his desire to keep the union and insisted he wanted to make a positive case for keeping Scotland in the UK. However, his list of issues raised – including Scotland losing its seat (as part of the UK) on the United Nations security council – could be seen by opponents as an attempt to threaten voters into shunning independence in the planned referendum.

Cameron said he would "fight with everything I have to keep our United Kingdom", describing it as "something really precious". He added: "To me this is not some issue of policy or strategy or calculation – it matters head, heart and soul."

He said his main argument would be "we are better off together".

He said: "We're stronger, because together we count for more in the world, with a permanent seat on the UN security council, real clout in Nato and Europe, and unique influence with allies all over the world.

"We're safer because in an increasingly dangerous world we have the fourth-largest defence budget on the planet, superb armed forces, and anti-terrorist and security capabilities that stretch across the globe and are feared by our enemies and admired by our friends.

"We're richer, because inside the United Kingdom Scotland's 5 million people are part of an economy of 60 million, the seventh-richest economy on the planet and one of the world's biggest trading powers. Today, Scotland has a currency which takes into account the needs of Scottish economy as well as the rest of the United Kingdom when setting interest rates and it can borrow at rates that are among the lowest in Europe."

Cameron is due to make a speech in Edinburgh before a meeting with Alex Salmond, Scotland's first minister and leader of the Scottish National party.

Although Downing Street said the PM did not intend to have detailed negotiations with Salmond on points of dispute about the referendum, his visit will be seen as a sign that he is getting personally involved in the discussions, officially led by the Scotland Office.

The main points of disagreement are whether the ballot will include a second option of the Scottish parliament having more power while remaining inside the UK; the role of the Electoral Commission in agreeing the wording of any questions; the timing of the vote; and whether the vote should be extended to 16- and 17-year-olds.

In a Scottish government consultation document published in January, Salmond suggested Scots should be asked the question: "Do you agree that Scotland should be an independent country?" He also said he wanted a referendum in May 2014.

In the most recent YouGov poll on the subject, a month ago, voters in Scotland were almost 60:40 in favour of giving Scotland more powers but the figures were reversed on full independence, with 39% for and 61% against.

Salmond, meanwhile, gave a speech on Wednesday last night arguing that, far from economic doom, independence would enable Scotland to "re-industrialise". Based on potential for tidal, offshore wind and wave power, plus remaining oil and gas reserves, in the first 20 years of independence, Scotland could generate an "energy fund" on the back of these resources worth £30bn, he told an audience at the London School of Economics.

"An independent Scotland would pursue policies of ambition and responsibility," he said. "We would use Scotland's natural resources and skilled workforce to build a sustainable economy. The rest of the UK has much to gain from the emergence of a secure, prosperous ally to its north."