British Prime Minister David Cameron has desperately urged British citizens to join his campaign to convince Scottish voters to vote "no" in tomorrow's referendum on Scotland's separation from the United Kingdom.

Meanwhile, Alex Salmond, the Scottish first minister, asked voters to push for independence.

As part of the "no" campaign, Cameron is sending emotionally charged emails to residents of England, Wales and Northern Ireland asking them to warn their friends in Scotland that "the future of our country is at stake."

"While only 4 million people can vote in the Scottish referendum on 18 September, all 64 million of us have a part to play in keeping our family of nations together," Cameron said in his email, sent on behalf of him by the Tory Party, the biggest partner of the UK government.

"It is a once-and-for-all decision. There is no going back. If Scotland votes 'yes', the United Kingdom will split, and we will go our separate ways forever," Cameron warned.

The latest polls predict a close call in the referendum.

"I desperately want our family of nations to stay together – and if you do too, please do everything you can to help save our great country," Cameron's message said.

The Tory Party has set up an online phone bank called "Connect 2015" and a special Facebook application called "Let’s Stick Together" – and called on British citizens to use them to reach out with the "no" message to their friends and family in Scotland.

"It would be heartbreaking to wake up on Friday morning to the end of the country we love," the prime minister said.

In his emotional appeal, Cameron described the UK as a country "that launched the Enlightenment, abolished slavery, drove the industrial revolution and defeated fascism.

"Millions of people in England, Wales, Northern Ireland and Scotland could not bear to see our country, the greatest country on earth, ending forever on Friday."

On the final day of campaigning, Salmond published a letter to the people of Scotland, telling them the power is in their hands as they vote to determine their country's future on Thursday.

"The talking is nearly done. The campaigns will have had their say. What's left is just us – the people who live and work here. The only people with a vote. The people who matter," the letter said.

Within the last few days, both independence supporters and pro-UK campaigners have held rallies to target the 8 percent of undecided voters – who could swing the vote either way.

With undecided voters excluded, polls suggest a win for "no" with 52 percent of the tally.