British Prime Minister David Cameron has made a heartfelt plea to the people of Scotland to remain part of the UK ahead of a referendum on the issue later this year.

Speaking in London, the Conservative Party leader made his most passionate defence yet of the UK, which comprises England, Scotland, Wales and Northern Ireland, warning a vote for independence would undermine Britain's global clout and imperil its financial and political stability.

If we lost Scotland, if the UK changed, we would rip the rug from under our own reputation.

David Cameron, UK prime minister

"We would be deeply diminished without Scotland," Cameron said, adding he would fight with all he had to hold the country he governs together.

Scotland will decide on September 18 whether it should end its 307-year-old union with the other nations and leave the UK.

Only four million Scots can vote, but Cameron's speech reached out to the 59 million people in England, Wales and Northern Ireland.

"If we lost Scotland, if the UK changed, we would rip the rug from under our own reputation," the Reuters news agency quoted him saying.

"We are quite simply stronger as a bigger entity."

Analysts say a "yes" vote would place the future of Britain's Scotland-based nuclear submarine fleet in doubt and could weaken London's claim to a permanent seat on the UN and its influence in the European Union.

"Together, we get a seat at the UN Security Council, real clout in NATO and Europe," Cameron said.

Tapping into an opinion poll earlier this month which showed the majority of people in England and Wales want Scotland to stay in the UK, Cameron urged the English, Welsh and Northern Irish to tell Scots: "We want you to stay".

'Up in the air'

Polls show increasing support for the "yes" campaign, led by the Scottish National Party (SNP), who are in power in Edinburgh, although the "no" vote remains ahead, with only around a third keen to break away from the UK.

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However, there are still many undecided voters and Cameron said the outcome was "up in the air," the Reuters news agency reported.

Alex Salmond, the leader of the SNP, criticised Cameron for giving such a speech in England instead of Scotland and challenged him to debate him, something Cameron has so far refused to do.

"I don't think this is a prime minister who speaks for Britain," Salmond said.

"The main thing is that this is a speech delivered from London, ostensibly telling people in England what to do but actually arguing against Scottish independence instead of a debate that the prime minister must do in Scotland."

Source: Agencies