Some of the world's biggest banks have gone on red alert on the increased chance that Scotland might secede from the United Kingdom.

A single poll released over the weekend has shown for the first time that a "yes" vote might succeed by a slender margin when four million Scots vote on September 18.

A second poll has now confirmed that the result was no fluke.

The surprise outcome has rocked British financial markets and the new uncertainty pushed the pound to its lowest level since November.

Evidence of a surge in support for Scotland breaking away has prompted some people to seek reassurance from banks that their savings are safe.

While banks have not reported evidence of withdrawals in the lead up to next week's vote, senior executives have held meetings with the Bank of England to discuss the possible scenarios.

In the event of a "yes" vote, banks are urging regulators to remind customers that Scotland will remain part of the United Kingdom for 18 months while the terms of the secession are negotiated.

Market complacency 'shattered'

The political, economic and social implications of a breakaway are massive, prompting global banks to warn clients that any transition might not be smooth.

Deutsche Bank: "Be afraid, be very afraid. The implications of a yes vote would be huge, and are magnified by the sense of institutional unpreparedness. A 'yes' vote could easily derail the UK economic recovery."

Goldman Sachs: "The near-term consequences of a 'yes' for the Scottish economy, and for the UK more broadly, could be severely negative."

UBS: "Significant risk" of bank deposits fleeing Scotland within days of a "yes" vote.

Citi: "With the lessons of the eurozone debt crisis still fresh in investors' minds, a currency union may weaken sterling in the same way it weakened the euro." Concerned that a Scottish exit will raise the chances of Britain leaving the EU within years.

Societe Generale: "Market complacency on Scotland is shattered."

As part of the caution, ratings agency Standard & Poor's has flagged that an independent Scotland might have trouble supporting its banks in the event of a financial emergency.

The Royal Bank of Scotland is considering the implications of a "yes", while Lloyds has confirmed it might move its registered office from Edinburgh to London.

Australian banks are not immune. National Australia Bank owns the troubled Clydesdale Bank.

The concerns of global banks are based on investment uncertainty surround the future of the British pound in Scotland and the tax and regulatory hurdles of Scottish independence.

Follow Peter Ryan on Twitter @Peter_F_Ryan and on his Main Street blog.