The company, which has worked on access bridges at football club Arsenal's Emirates Stadium, said last month it anticipated full-year results to show a marginal improvement over 2011.
"While we still expect to deliver profit growth year on year, profit for the full year may be lower than current expectations," the company, which has also worked on Wimbledon's Center Court and the 2012 Olympics stadium, said in a statement.
The company expects cost overruns of £1.6m and first-half pretax profit of £1.5m.
The company posted a pretax profit of £3.4m for January-June last year.
A company spokesman declined to name the projects affected but said they remained on track for completion in line with customers' timetables.