Confirming the merger, which will create a £400m, 7000-strong multidisciplinary practice to rival WS Atkins, the firms said the deal could be a wake-up call for the sector.
Martin Bishop, chairman of Franklin + Andrews, said: "It will make the engineering firms think whether they want to do something similar. It will also make the big QS firms think about their future. They may choose to do nothing about it but they will still think."
Bishop said the deal followed the logic laid out in Sir John Egan's Rethinking Construction report.
He said: "A merger between two QSs would have been more of the same. Egan showed that the problem with the industry was that it was too fragmented. The right way has to be to pull different skills together to offer a one-stop shop."
The merger, first revealed in Building last week, comes after nine months of talks between the two firms. Mott MacDonald chairman Tim Thirwall said the group had been looking for a QS for some time, but denied the firm had been in talks with Franklin + Andrews rival, Gardiner & Theobald.
Thirwall said: "Last year was a great one for both of us in terms of growth and profitability. It's always good to do it when both firms are strong."
The deal will result in Franklin + Andrews forming part of the Mott MacDonald group. Bishop and Franklin + Andrews managing director Guy Leonard will join the Mott MacDonald board.
Both firms said they would keep their names and those of their subsidiaries. Bishop said: "We are not ready to rush into any changes just yet. We will take a view as to what's best for the market and the organisations."
The firms stressed that there would be no redundancies as a result of the deal, and Thirwall said there were no plans for the enlarged group to go public.