Prime Minister David Cameron made a passionate case for Scotland remaining part of the United Kingdom on Thursday as he prepared to meet Scottish leader Alex Salmond for talks on a referendum that could see Scotland breaking away.

The fight is now under way for something really precious: the future of our United Kingdom. I will fight with everything I have to keep our United Kingdom together, Cameron said in excerpts of a speech he will give later on Thursday in the Scottish capital Edinburgh.

To me, this is not some issue of policy or strategy or calculation - it matters head, heart and soul. Our shared home is under threat and everyone who cares about it needs to speak out, he said.

Cameron and Salmond, first minister of Scotland's devolved government, will hold talks in Edinburgh on Thursday to thrash out differences over a referendum promised by Salmond that could lead to Scotland ending its 300-year-old union with England.

Salmond, who heads the pro-independence Scottish National Party (SNP), wants a vote in late 2014. Cameron's government, which opposes a breakaway, wants the poll sooner to avoid damaging uncertainty for Scotland's economy.

The SNP would also like to offer Scots the option of voting for greater devolution, a half-way house between the status quo and independence. The government favours a simple yes or no question on independence.

Polls suggest between 30 and 40 percent of Scots support independence. The SNP hopes it can increase that by 2014, when national pride may be boosted by the 700th anniversary of the Battle of Bannockburn, a famous victory over the English.

DILEMMA FOR CAMERON

All major British parties want to keep the union intact, but Cameron faces a dilemma over how to handle the pro-union campaign because his Conservative Party is unpopular north of the border, where it has just one member of parliament, and Salmond portrays Cameron's interventions as interference.

Some battle lines have already been drawn, with the SNP demanding 90 percent of Britain's North Sea oil revenues for Scotland while also arguing that the Bank of England should rescue distressed banks in an independent Scotland.

British taxpayers stumped up billions to save Royal Bank of Scotland from collapse during the 2008 crisis.

An independent Scotland would have to choose whether to keep using the British pound while having no influence on British monetary policy or to seek to join the euro at a time when the single currency zone is in turmoil.

Of course, there are arguments that can be made about the volatility of dependence on oil, or the problems of debt and a big banking system. But that's not the point. The best case for the United Kingdom is entirely positive. We are better off together, Cameron said.

Cameron argued that Scotland, England, Wales and Northern Ireland, which make up the United Kingdom, were stronger together because of Britain's permanent seat on the United Nations Security Council and its clout in NATO and Europe.

They were safer because Britain had the world's fourth-biggest defence budget and they were richer because Britain's economy was the world's seventh biggest, he said.

Salmond said an independent Scotland would use renewable energy to help build a sustainable economy. It potentially has a quarter of Europe's offshore wind and tidal power, he said.

We would use Scotland's natural resources and skilled workforce to build a sustainable economy, he said in a speech in London late on Wednesday.

The rest of the UK has much to gain from the emergence of a secure, prosperous ally to its north.

(Additional reporting by Drazen Jorgic; editing by Andrew Roche)