Flint & Neill has 50 years experience as an independent private consulting practice, while COWI is well known worldwide for its work on major long span bridges.
"We will carry on providing services but with COWI backing, which means extra security, skills and resources," said Flint & Neill partner Ian Firth.
"COWI has been interested in us for some years. Now the time is right, we can form a partnership."
Flint & Neill partner David MacKenzie along with Firth will continue to lead the firm, with MacKenzie taking on the role of chief executive officer and Firth continuing as chief operating
COWI and Flint & Neill have worked together previously notably in the Lusail bridge in Qatar. They are currently part of a joint bidding for the Fehmarn Belt bridge which will connect Germany with Denmark.