The merger, completed this week, means the enlarged firm has a turnover of £80m with a staff of 1400.
All the partners in Povall Flood Wilson, which specialises in private finance initiative projects in education and retail work, will join EC Harris as partners when the firms combine forces in May. Povall Flood Wilson's name will no longer be used unless in conjunction with the EC Harris brand.
EC Harris chairman Richard Clare said the firm was bullish about the future: "Our aim is to be the leading firm of its type on a global basis.
"Most of our growth has been homegrown until now, but this was so appealing because it adds so many high-quality people to our firm."
Povall Flood Wilson chairman Alan Brookes added: "Our ability as a joint force to invest in developing a group that services the whole asset life-cycle with high-quality resources provides massive opportunities for all involved."
Clare said the pair had been in talks since before Christmas. He said: "Its aims and ideals ran closely with our own, which was unusual compared with all the other firms we have talked to before."
Offices in Liverpool, Manchester, Cardiff, and Glasgow, where both firms have branches, will be amalgamated. The combined group has 32 offices throughout the world.
Clare said there were no plans for big job cuts: "I cannot imagine many more than 10 jobs will be lost. We are not using this as an excuse to get rid of anyone."
He added that he thought the deal was a "perfect fit", both geographically and in terms of markets: "Povall Flood Wilson is very strong in education, where we are not. It also has operations in the North-west and South Wales, where we were looking to grow. It is a win-win situation.
"We are using our own brand, but if there is a particularly strong part of Povall Flood Wilson, then we will incorporate this with the EC Harris brand. We don't want to throw the baby out with the bathwater."
Povall Flood Wilson was formed in 1997 after Povall Worthington and Flood & Wilson merged. It has 260 staff.