(Adds comments from letter organiser)
By Paul Sandle
LONDON, Aug 27 (Reuters) - More than 130 business leaders came out against Scottish independence on Wednesday, highlighting the economic risks of secession days after the campaign to end the union was boosted by a strong performance in a TV debate.
The heads of giants BHP Billiton, temporary power provider Aggreko and HSBC bank joined forces with the bosses of smaller firms within Scotland's whisky and fishing industries to sign a letter calling for the United Kingdom to stay together.
The letter opens a new front in the campaign in Scotland just three weeks before the vote. Many companies, large and small, had previously refused to take sides in the highly-charged debate.
Independence campaigners argue Scotland could flourish on its own, with Edinburgh taking more direct control over economic development and social welfare and drawing greater benefits from North Sea oil.
Consultant Amanda Harvie, one of the organisers of the letter, said the signatories had concluded independence would not provide the strong platform needed to keep Scotland flourishing, and the point needed to be made.
"There is ongoing uncertainty around issues that are of critical importance to business," she told Reuters on Wednesday.
"Bearing in mind our economy is performing well at the moment, and we are succeeding as part of the United Kingdom, it would not be a good idea to change the positive platform that we have on which to build."
The former chief executive of industry body Scottish Financial Enterprise, Harvie said she and the other organisers felt it was vital to make their views known ahead of the vote on whether Scotland should end its 307-year-old union with England and leave the United Kingdom.
Some major companies, such as BP and Shell, have said they would prefer Scotland to remain in the United Kingdom because of the uncertainty that a vote to leave would cause.
Others have not voiced a view on a subject that divides the Scottish electorate, and therefore their customers.
Harvie said she met many companies who supported the union with England but didn't feel they were in a position to publicise it, particularly if they were involved in government contracts.
But she added that whilst some companies had declined to sign, she had not talked to a single company that thought independence was a good idea.
On the other side of the debate, Michelle Thomson, managing director of Business for Scotland, a co-operatively owned business network for pro-independence business people, told the BBC that staying within the union also presented businesses with uncertainty given a possible future referendum on Britain's membership of the European Union.
The intervention from business is likely to provide a shot in the arm for the British government's "Better Together" campaign after its leader struggled in a recent TV debate with the head of the pro-independence movement, Alex Salmond.
Several recent polls have shown support for independence pushing higher, but the most recent "poll of polls", on Aug. 15, which was based on an average of the last six polls and excluded undecided respondents, found support for a breakaway stood at 43 percent against 57 percent for remaining within Britain.
On Wednesday, former British prime minister Gordon Brown joined forces with Alistair Darling, the head of the "Better Together" campaign and his former finance minister, to call on voters to reject independence.
If Scotland, with its $250 billion (150 billion pound) economy, 5.2 million people, oil industry, and nuclear submarine base, leaves Britain, with its $2.5 trillion economy and 63 million people, the consequences would be profound.
Britain's three main political parties want it to stay in the union, which includes England, Wales and Northern Ireland.
"This is about business informing the debate," said Keith Cochrane, chief executive of Scotland-based engineering company Weir Group and another organiser of the letter.
"What we are seeking to do is highlight the concerns of real businesses, business with tens of thousands of employees, businesses representing all facets of Scottish business life to ensure those concerns can be taken on board by the voters,"
All signatories had signed the letter in their personal capacity.
Reporting by Sarah Young in London and Karen Rebelo in Bangalore; Editing by Angus MacSwan