Janet Yellen will deliver the biggest shock to markets since taking over as chair of the Federal Reserve should the central bank raise interest rates this month, according to a survey of Wall Street economists that shows more than 85 per cent expect it to hold fire.

The scepticism among economists may concern the Fed’s top officials, who have spent the past month trying to persuade financial markets that an increase at their meeting on September 21 is a possibility given that the US unemployment rate is below 5 per cent and the global fallout from the Brexit vote has been muted.

They have succeeded at least in jolting financial markets out of months of complacency, after comments from two voting members on the Fed’s policy-setting committee on Friday sent US stock and bond prices sliding at their greatest rate since June.