By noon on Wednesday, Interserve's share price was 230p, compared with an opening price of 342p – a fall of 33%. Analysts were treating the statement from Bottjer as an unofficial profit warning because, traditionally, Interserve performs better in the second half of the year.
Interserve's shares fell despite the group reporting a 33% jump in pre-tax profit to £19.9m for the six months to 30 June.
Group turnover increased by £13m to £568m.
Interserve's downbeat statement dented Carillion's share price on Wednesday, even though it nearly doubled its profit in the first half of the year. Pre-tax profit surged 96% to £16.1m, but the share price dropped 7p to 151p.
Carillion chief executive John McDonough was bullish about his firm's prospects. He said: "Continued success in our chosen markets has increased our record order book to £5.3bn."
McDonough added that PFI bidding activity remained high.