The tie-up, which industry sources claim will be agreed next week, will cement Mott MacDonald's position as the UK's second largest consultant. It could also spark a wave of consolidation among UK consultants.
News of the deal emerged this week, and comes after speculation over the firm's expansion plans. It is understood that Mott MacDonald was looking at a similar deal with QS Gardiner & Theobald.
The merged firm would have a total workforce of nearly 6000 – still less than half the number at WS Atkins, which has 13,000 staff worldwide and a turnover of £674m.
Both firms have expanded in the past two years. Mott MacDonald announced an alliance with Australian engineer Connell Wagner last February, creating an arm called Connell Mott MacDonald to target markets in Europe, Asia and Australia. Mott MacDonald has about 150 offices in Europe, Asia, the Middle East and the Americas.
Franklin + Andrews' project management arm Osprey Project Management acquired Project Management International from BAE Systems in 2000 to expand its area of operations to continental Europe. The company, established in 1866, was shortlisted as surveyor of the year in this year's Building awards.
The merger is viewed as similar to the acquisition of QSs Faithful and Gould and Silk & Frazier by Atkins in 1996 and 1998. Building revealed in March that Atkins was in talks with QS Hanscomb to take over its US operations.
Industry sources claim that the deal could lead to a wave of similar mergers between QS practices and engineering consultants to create firms that can offer a wider service over a larger area.
One rival QS, however, denied the move would lead to a rash of similar deals.
The QS said: "The big boys such as Davis Langdon & Everest and EC Harris are still safe, and have diversified anyway.
"The Mott deal is a model for the future, but not the only model."