Bosses at 130 of Britain's largest companies have signed a letter that stipulates that the business case for Scottish independence "has not been made."

According to the letter, published in The Scotsman newspaper, companies such as HSBC, BHP Billiton, and BG Group, say that "much is at stake" as there is still great uncertainty surrounding a number of issues, if Scotland becomes independent.

"Our economic ties inside the United Kingdom are very close and support more than a million Scottish jobs," said the letter.

"As job creators we have looked carefully at the arguments made by both sides of the debate.

"Our conclusion is that the business case for independence has not been made."

Scottish people will vote in an independence referendum on 18 September, 2014, and will be asked the straight "yes/no" question: "Should Scotland be an independent country?"

Concerns have arisen over the lack of clarity on issues such as currency, tax, EU membership and support for exports.

The 130 company executives, which signed the letter, lead firms that are responsible for about 50,000 jobs in Scotland.

One of the signatories, HSBC chairman Douglas Flint, has already warned that Scottish independence will leave the country's financial system in a "parlous state" as severing the 307-year old union will lead to a capital flight.

Meanwhile, Keith Cochrane, chief executive of Glasgow-based engineer Weir Group, which organised the letter had previously commissioned an independent report to cut through "political rhetoric" and analyse the true impact of a union break.

However, pro-independence lobby group Business for Scotland said:

"Business for Scotland has 2,500 members who run businesses in Scotland, employ people across the country in a range of industries, and all believe that Scottish independence is in the best interests of Scotland and Scottish business.

"It's a position reached after looking at the facts and figures and realising that, from a simple balance sheet point of view as well as other considerations, our best interests lie in becoming an independent country."