Vinci Plc, which also has a number of PFI investments, made the comments on the back of improved results in 2007.
Pre-tax profits jumped 27 per cent to £40.4 million. Turnover rose 22 per cent to £774 million.
But finance director Alec Comba said its private sector workload was coming under pressure from clients taking more time to part with their money.
He said: “We are seeing a slowdown in the rate of -conversion from preferred bidder to a start on site. We are seeing a lot of evidence that the private sector is closed for business.
“In terms of volume, we’re seeing fewer new orders than the ones we’re completing.”
The firm said the number of distribution depots it had been asked to build had fallen and Mr Comba said the problems dated back to the last quarter of 2007.
He added: “The public sector will become vitally important. We have got a number of pending orders from the public sector over the next 12 to 18 months.”
The firm’s businesses mainly carry out health and education deals in the public sector.
Mr Comba said it would continue with its policy of picking up regional contractors as part of a strategy to build up a network around the country.
Last year it bought West Midlands company Weaver and earlier this year completed the takeovers of Cardiff-based business Stradform and Newcastle upon Tyne social housing specialist Gordon Durham.
Norwest Holst makes up two-thirds of Vinci Plc’s turnover and last year revenues rose one fifth to £490 million thanks to a strong performance from its building division.
But pre-tax profits slipped 13 per cent to £16.8 million after its earthmoving business John Jones was held up carrying out work on a road job in Ireland for Turkish contractor Gama.