Six months before Scotland votes in a referendum on whether to become independent from the rest of Britain, major companies are going public on their plans whatever the outcome. Firms ranging from Royal Bank of Scotland to British Airways and Royal Dutch Shell are stating their positions ahead of Scotland voting on whether to end its 300-year-old union with England. British PM David Cameron's Conservatives, their coalition partners the Liberal Democrats and the opposition Labour party have joined forces to campaign for a "no" vote. But First Minister Alex Salmond's governing Scottish National Party is in the "yes" camp and wants Scotland's 5.3 million people to quit the UK. Scotland has enjoyed increased autonomy since a 1997 referendum on devolution but the SNP now wants full independence. Some companies have said they would be opposed to an independent Scotland, but the overriding sentiment is one of concern about the uncertainty. The referendum in September is creating uncertainty for our customers and our business, which we have a responsibility to address," said Katherine Garrett-Cox, chief executive of Scotland-based investment manager Alliance Trust. "Regardless of the outcome, it is critical that we are able to provide continuity of service and protection for their investments and savings. "To give them full confidence, we have started work to establish additional companies registered in England, in order to provide operational flexibility and to complement our existing business in Scotland." Also last week, financial services group Standard Life warned that it may move some of its operations out of Scotland if it votes for independence. - RBS predicts major impact - RBS added that there would likely be a "significant impact" if Scotland chose to end its union with England, placing renewed pressure on Salmond ahead of the vote. A key issue in the debate is the currency of an independent Scotland, with Edinburgh saying it would keep the pound sterling but London saying that would not be possible. "We have started work to establish additional registered companies to operate outside Scotland, into which we could transfer parts of our operations if it was necessary to do so," said Standard Life chief executive David Nish. Nish said several key issues "remain uncertain" in case of independence, including the currency and the financial services regulations of an independent Scotland and its future within the European Union. British state-rescued RBS said uncertainty over Scottish independence posed a financial risk. "A vote in favour of Scottish independence would be likely to significantly impact the Group's credit ratings and could also impact the fiscal, monetary, legal and regulatory landscape to which the Group is subject," it said. "Were Scotland to become independent, it may also affect Scotland’s status in the EU." - Shell, BA at odds - While Standard Life's Nish said the company had a "long-standing policy of strict political neutrality", the chief executive of oil giant Royal Dutch Shell dealt a blow to the campaign for Scottish independence. Ben van Beurden explained this week that independence would introduce greater uncertainty into the North Sea oil industry, a crucial source of income for Scotland and the Anglo-Dutch company. For similar reasons, van Beurden said he wanted Britons to vote to stay in the European Union during a possible referendum on its membership in 2017. "Shell has a long history of involvement in the North Sea -- and therefore in Scotland -- and we continue to invest more than a billion pounds ($1.67 billion, 1.21 billion euros) there every year," he said. However, Willie Walsh, chief executive of British Airways owner IAG, has declared that a 'yes' vote could be a "positive development" and might lead to the Scottish government abolishing air passenger duty (APD). Michael O'Leary, chief executive of Irish low-cost airline Ryanair, added he supported the abolition of APD to boost traffic and tourism, but has declined to enter the independence debate. "Speaking as an Irishman, that's a matter for the Scottish people," he told the BBC. "But certainly, if the air travel tax were repealed by the UK government or an independent Scottish government, you'd see visitors to Scotland double over a five- to 10-year period."