The British Prime Minister David Cameron is to make a final plea to Scots to reject independence and keep the Union together on his last visit north of the border before Thursday's referendum.

As polls continue to show both the 'Yes' and 'No' camps neck and neck ahead of the vote, the two sides are pulling out the stops to rally support among undecided voters.

Scottish First Minister Alex Salmond will be meeting business leaders - including Brian Souter of Stagecoach and former William Hill chief executive Ralph Topping - in an effort to highlight the economic opportunities of Scotland going it alone.

It follows growing warnings from business and financial chiefs about the impact independence could have, with stores such as John Lewis and Asda last week suggesting prices could have to rise, while Deutsche Bank likened a vote to leave the UK to the mistakes which led to the Great Depression of the 1930s.

And today, according to a survey, 65% of 200 of the City of London's top investors believe Scotland's economy is at risk if it votes 'Yes'.

Research by spread betting company Capital Spreads found that 65% of investors expect Scotland's economy to be weaker five years after independence, falling to 63% after 10 years.

When Mr Cameron addressed financial services workers last week he told them it would break his heart if Scots voted 'Yes' and split apart the UK "family of nations".

But on today's visit he is expected to issue a stark warning towards waverers that there could be "no going back" if Scotland backed independence.

'Once in a lifetime opportunity'

Mr Salmond has said the referendum could be a "once in a lifetime opportunity" for people in Scotland.

Latest polls put the 'No' campaign ahead - with a survey by Panelbase giving them a lead of just over one point when undecided voters are excluded.

The poll, for the Sunday Times, put support for 'No' at 50.6%, narrowly ahead of 'Yes' on 49.4%.

A rival ICM poll recorded an eight-point lead for the pro-independence campaign - but experts urged caution because of its smaller than normal sample size. Mr Salmond has said the 'Yes' campaign was aiming to win a "substantial majority" in the referendum.

The First Minister said: "My view is this is a once in a generation, perhaps even a once in a lifetime, opportunity for Scotland."

Mr Salmond said work was already under way to assemble a team of specialists to negotiate terms with the rest of the UK in the event of a vote for independence.

Meanwhile, footballer David Beckham has added his voice to the campaign against Scottish independence, urging voters north of the border not to ditch a bond which was "the envy of the entire world".

But fashion designer Vivienne Westwood used her catwalk runway on Sunday to urge Scots to break away from the UK by pinning 'Yes' badges on all her models.

Speaking backstage, she said: "I hate England. I like Scotland because somehow I think they are better than we are. They are more democratic."

It came after Queen Elizabeth II said she hoped people would "think very carefully about the future" as the referendum campaign enters its final days.

And the former UK Labour prime minister Gordon Brown has revealed that a House of Commons debate on increased powers for the Scottish Parliament has been set for October 16th.

The ex-Labour leader recently unveiled his plan for a timetable to give the Scottish Parliament more control over finance, welfare and taxation as part of a new Scotland Act if voters reject independence.

Dr Matt Qvortrup is a referendum expert, academic and author. He told Newstalk Breakfast that the gap of the undecided is narrowing.