The order book is made up of over 90% repeat orders from 'blue chip' customers.
More than £700m of revenue is already secured for 2014, an increase of over £100m on the equivalent figure last year. In excess of £1.8bn is secured for 2015 and beyond.
Revenue for the first half of this year was £462.9m, down slightly from last year's £477.9m, but the underlying profit grew from £10.4m to £10.7m. The adjusted profit before tax was £8.4m.
Chairman David Allvey said: "This was an encouraging start to the year with a 20% increase in our order book to £2.9 billion and an increase in underlying operating profit, against a backdrop of market conditions which continue to be challenging.
"Costain is building its order book by generating, through its commitment to 'Engineering Tomorrow', innovative integrated service solutions for major customers who are continuing to invest to address essential national needs."
In the first half of the year, Costain was awarded new contracts and extensions worth more than £900m to the group.
- The £450 million AMP6 programme for Thames Water, our share of a landmark £1.2 billion contract secured in joint venture;
- A £300 million contract, in joint venture, to design, fit-out and commission the railway systems for Crossrail;
- The FEED design for Centrica's gas terminal at Barrow, following successful completion of the design and construction of the Easington terminal for the York field;
- A highways framework contract for Transport for London including the Hammersmith Flyover strengthening project; and
- The electrification upgrade of the West Coast Mainline for Network Rail - a new area of operation for Costain.
Revenue in the Infrastructure division during the period was £262.8m (2012: £246.2m), with adjusted profit from operations of £14.4m (2012: £9.6m). The order book for the division has grown to £1.7bn from £1.5bn.