Billions of pounds were clipped from the value of FTSE-listed giants who do business from Scotland after an influential pollster said more people were planning to vote for independence than against in the impending referendum.

Towards the end of trading on 8 September, the FTSE 100 had dipped as the share prices of RBS, Lloyds Banking Group, Standard Life, Weir Group, and SSE all tumbled. Sterling had plunged against the dollar, euro, and yen.

It followed news in the morning that 51% of people polled in a YouGov survey said they would be voting for independence in the Scottish referendum on 18 September; the first time the 'Yes' campaign had the upper hand and a sign that the UK may be about to break apart.

Markets and companies are worried about the uncertainty of independence, with major questions – such as what currency Scotland would use and what its financial regulatory system would look like – still unanswered.

Some firms headquartered in Scotland, such as Standard Life, have suggested they would leave the country in the event of independence.

Alex Salmond, Scotland's first minister and leader of the Scottish National Party (SNP), says the country would carry on using the pound and the Bank of England as its central bank as part of a currency union with the rest of the UK. He also says Scotland would join the European Union (EU).

But the major Westminster parties have all said there is no guarantee an independent Scotland would be allowed to enter a formal currency union.

EU officials have said it is not a given that Scotland would join the trading bloc. There is nervousness in countries with independence-minded regions, such as Spain, that accepting Scotland into the EU as a newly independent state would raise difficult political questions for them at home.

"The implications of a yes vote would be huge," said Oliver Harvey, a forex analyst at Deutsche Bank, in a research note.

"On the currency side, it could at worst lead to a destabilising crisis in the whole British banking system and at best leave the rest of the UK with an unstable currency union."

Moreover, Harvey said that Scottish independence could "easily derail the UK economic recovery" because Scotland is the rest of the UK's largest trading partner after the EU "and many corporate investment plans are likely to be put on hold until clarity over currency, regulatory and tax questions is achieved".