Scotland's First Minister Alex Salmond.

London - More than 130 company bosses have come out against Scottish independence, saying the business case for it has not been made, in one of the biggest corporate interventions in the referendum debate just three weeks before the vote.

Chief executives of FTSE 100 companies including mining giant BHP Billiton and temporary power provider Aggreko plus the chairman of HSBC bank, and bosses of whisky, fishing and potato firms all put their names to a letter calling for the United Kingdom to stay together.

Voters will decide on September 18 whether Scotland should end its 307-year-old union with England and leave the United Kingdom.

The latest intervention against independence is likely to provide a shot in the arm for the British government's “Better Together” campaign and comes as the pro-independence side struggles to catch up in the polls, despite its leader Alex Salmond's recent success in a TV debate.

Several recent polls have shown support for independence pushing higher, but the most recent “poll of polls”, on August 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.

The open letter published on Wednesday said that as “job creators” the companies had studied both sides of the argument.

“Our conclusion is that the business case for independence has not been made,” it said, adding that uncertainty surrounds issues such as currency, regulation, tax, pensions, EU membership and support for exports.

If Scotland, with its $250 billion (R2.7 billion) 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.

The letter, organised by Keith Cochrane, chief executive of Scotland-based engineering company Weir Group, said that with Scotland part of the United Kingdom, businesses have a strong platform for investing in jobs and industry.

“This is about business informing the debate,” Cochrane told the BBC on Wednesday.

“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.

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. - Reuters