Optimism among Britain’s top business executives rebounded to an 18-month high at the end of 2016, but they say they are less willing to take risks in 2017 because of uncertainty surrounding Brexit.

A Deloitte survey of chief financial officers (CFOs) at some of the UK’s biggest companies suggested that confidence had risen because of the unexpected resilience of the economy following the EU referendum in June.

The fourth-quarter poll, however, also found that CFOs do not believe it is a good time for their firms to take financial risks, with the effects of Brexit topping their list of concerns.

Ian Stewart, Deloitte’s chief economist, said: “Buoyed by a backdrop of continued UK growth, CFOs have become markedly more positive on the outlook for their businesses and enter 2017 in better spirits than at any time in the last 18 months.

“However, rising optimism does not represent a return to business as usual. Uncertainty in the external environment continues to keep a lid on corporate expansion. Risk appetite remains depressed and is well below average levels and corporates remain on a defensive footing, with cost reduction and building up cash as their top priorities.”

Immediately after the referendum, optimism plunged to the lowest level since the 2008 crisis as finance bosses grew fearful of the consequences of the shock result. Sentiment started to recover when the economy proved more resilient than expected in the months following the vote, with growth of 0.6% in the third quarter matching the rate in the second quarter.

In the latest survey, 27% of CFOs said they were more optimistic about prospects for their companies than they were three months ago, while 22% said they were less optimistic. It was the first quarter since the second quarter of 2015 where a net balance of CFOs were more optimistic.

Concerns are mounting, however, about the impact Brexit will have in the future. Of the 119 CFOs of FTSE 350 and other large private companies Deloitte surveyed, 89% said they were facing high levels of economic and financial uncertainty, up slightly from 88% in the previous quarter but down from 95% in the immediate aftermath of the EU referendum.

David Sproul, Deloitte chief executive and a senior partner, said: “2016 has been a tumultuous year, but the optimism seen in this survey does give us some confidence heading in to 2017. Uncertainty is the new normal in business life, but the challenge for corporates is to gauge and mitigate external risks while staying open to opportunities.”