Costain is to merge with May Gurney.
Costain earned pre-tax profits of about £26 million on revenues o f£930 million in 2012. May Gurney had revenues of about £700 million in its latest financial year, earning profits of #19.3 million.
Costain's existing shareholders will take 53% of the enlarged company, with May Gurney investors taking 47%.
The deal still needs the backing of both sets of shareholders.
Costain and May Gurney have announced merger plans.
The companies will have combined revenues of £1.6 billion and challenge rivals including Balfour Beatty and Carillion.
Major infrastructure contractor Costain, which is working on London's Crossrail underground rail tunnel, is based in Maidenhead, which will be the group's headquarters.
The group plans to retain May Gurney's headquarters in Trowse, Norwich, as a shared services base.
May Gurney has around 800 of its workforce in Norfolk.
Construction firm Costain has unveiled a £370 million merger with road and rail maintenance firm May Gurney.
About 150 jobs will be cut from a combined workforce of 11,000 as the group looks to save £10 million a year from the all-share deal.