The City remains confident that the UK is on course to remain a member of the EU despite a late flurry of sell orders after a new opinion poll boosted Brexit supporters just as the London stock market closed.

After a day in which shares and the pound had both risen amid growing confidence that David Cameron’s gamble to call the vote would pay off, there was a last-minute wobble following the publication of an Opinium survey that gave the leave campaign a marginal lead.

The FTSE 100 had been on course for its highest level for two months, up more than 70 points, before closing just 35 points higher at 6261. The pound, seen as a barometer for confidence in UK assets, had been up a cent against the dollar but lost half a cent after the poll came out to trade at around $1.47.

Despite the late sell-off, the FTSE has regained the losses it sustained last week when the City started to take fright that a Brexit was on the cards. City dealers expect the FTSE to plunge if both they and the bookmakers are wrong about their assumption that the referendum will back staying in the EU.

According to predictions by a UBS strategist, Yianos Kontopoulos, £350bn could be wiped off the FTSE 100. He forecast that the index could fall as low as 4900 in the “worst case” after a Brexit vote, the lowest level since the height of the eurozone crisis in 2011. But in the event of a vote to remain, the index could reach 6800, he said.

“Following the risk-rally of the last few days, we believe there is significant room for downside in the event of a leave vote,” Kontopoulos said.

Brokers said there had been a correlation between the FTSE and the polls in recent days.

Laith Khalaf, senior analyst at brokers Hargreaves Lansdown, said: “In the long run the jury is out on what Brexit means for the stock market, given the number of variables involved. However based on how markets have been behaving of late we do expect an immediate relief rally in the event of a Bremain vote, and a fall in the event of a Brexit.”

Sterling’s value is being scrutinised, and analysts at UBS said the pound, in the event of a Brexit, could devalue against the euro so that up to 89p would be needed to buy a euro, compared with 77p now.

This week, the investor George Soros sparked a debate by warning, in a comment article in the Guardian, of a Black Friday occurring after a vote for Brexit that would trigger a 20% fall in the value of the pound, taking it to below $1.15 and parity with the euro.

But Albert Edwards, global strategist at the French bank Société Générale, took issue with Soros’s assertion that this would be bad for the economy. Edwards said in a research note to the bank’s clients that a reduction in the value of the currency would be as beneficial as it had been during the period after Black Wednesday in September 1992 when Soros’ speculative attack on sterling drove the UK out of the exchange rate mechanism (ERM).

Warning that the pound could fall whatever the outcome of the referendum, Edwards said: “There is an argument that a Brexit might look similar to the aftermath of sterling’’s ignominious exit from the ERM. After [that] much feared event the UK economy actually recovered strongly and unemployment fell sharply.

“In a current environment, where central banks and governments have failed to generate a strong enough economic recovery to normalise interest rates amid persistent deflationary pressures, one would have thought a substantial decline in one’’s currency would be welcomed – –for that is one way to inject a modicum of inflation back into the economic system.”

Britain ran a current account deficit – on its trade and overseas investments – of 5.5% of GDP last year and the budget deficit, the gap betweem government expenditure and what it raises in taxes, stood at 4% of GDP despite attempts by George Osborne to balance the books over the past six years.

“The UK economy is a mess and that has nothing to do with Brexit, it has everything to do with economic mismanagement,” Edwards said. “I think sterling will end up falling substantially whether the UK stays or leaves the EU.– It is just a matter of timing.”