The UK’s infrastructure spending boom was already fizzling out before the Brexit vote, according to a survey of civil engineering firms that shows a slowdown in activity and order books leading up to the EU referendum.

In the latest evidence that the referendum has hit business confidence and investment plans, there was also a drop in sentiment and hiring expectations among the companies that build and maintain infrastructure such as roads, railways and the energy supply.

A poll by the Civil Engineering Contractors Association showed that six out of 10 subsectors reported falling workloads in the three months to the end of June, compared with the same period a year earlier. From the 101 companies surveyed, a net balance of 2% said workloads had risen in the latest quarter, a sharp decline from 20% in the previous quarter and the weakest result for three years.

CECA’s chief executive, Alasdair Reisner, said the recovery in infrastructure activity appeared to have been cut short. “Right now we are supposed to be going hell for leather on infrastructure. There is a substantial programme of private and public investment that was supported by government ... We thought that from here on in, we were going to see speeding up of activity,” he said.

“These results spell real trouble for the UK economy and should act as a major warning sign to policymakers. We know that infrastructure investment is a driver of economic growth. Given the recent disappointing economic forecasts following the Brexit vote, our figures show that the market is slowing just as the country needs it to speed up.”

The downbeat poll results, 85% of which were collected after the referendum result was confirmed on 24 June, echo the cautious tone among manufacturers in a survey by the CBI published on Monday, which showed that optimism had slumped to its lowest level since the depths of the global recession.

The first official estimates of how the economy fared in the run-up to the referendum are released on Wednesday. Figures for second quarter GDP from the Office for National Statistics are expected to show that growth held steady at a relatively muted 0.4%, according to a poll of economists by Reuters.

For his sector, Reisner said, the biggest threat from a drop in confidence was to privately funded infrastructure projects. “I don’t think it will be the public sector works where the challenge is. The issue will be the private sector taking another view and asking if the market is there or not, and wondering what will happen to the wider economy,” he said.

CECA, whose 300 members account for about three-quarters of UK civil engineering activity, said there had been a slowdown in orders. The net balance of surveyed firms reporting an improvement in order books fell to 9% in the second quarter, down from 13% in the first quarter, and the lowest reading since the end of 2013.

Reisner said a slowdown in infrastructure spending and activity would impact the economy in several ways. There was a potential blow to hopes of rebalancing growth in the UK if building projects in the north of England were affected, he said. There were also ramifications for the UK’s overall economic growth and for jobs.

Construction hit hardest in last downturn

He said the lesson of the last recession, which hit the construction industry harder than any other sector of the UK economy, was that continuing to spend on infrastructure could safeguard jobs. “Unless the government kicks on to get spades in the ground, we will be looking at a dramatic slowdown in growth, which is bad news for the 200,000 people who work in our sector, and bad news for the economy as a whole,” Reisner said.

CECA does not expect any near-term drop in employment, given that most civil engineering firms would likely want to hold on to experienced workers amid a continuing skills shortage. Employment expectations for the next 12 months were the weakest for more than three years.