The firm’s 183 partners voted overwhelmingly in favour of the deal, with all but one backing the merger.

The two firms can now go ahead and merge the business as planned this Friday.

Philip Youell, chief executive of EC Harris, said: “I am delighted that our partners have ratified the proposals and we can now proceed to merge with Arcadis.

“Arcadis and EC Harris are a natural fit and there is genuine excitement about the opportunities that the merger presents.

“We will now begin the process of integrating our two firms, with EC Harris retaining its brand within the wider group.  I am looking forward to working with our new colleagues at Arcadis to meet our clients’ needs.”

The merged company will boast almost 19,000 professionals and a €2.3bn turnover.

Arcadis will leapfrog its way through the rankings to become a top five global leader in building consultancy and project delivery services, with close to 6,000 staff in Europe, the US and Brazil.

The deal brings Arcadis into the top five consultants in the UK, allowing it to expand its relationships with multinational clients both inside and outside the country.

EC Harris will continue to operate as a partnership, with a company-wide remit within Arcadis, taking up a leadership role in driving the development of programme and project management services.