The American offer has trumped a £591m bid by consultancy giant WSP which was all set to go through after being announced in August.

But Tetra Tech is now set to be the new owner of  RPS who employ consultants, designers, planners, engineers, and technical specialists across the world providing specialist services to government and private sector clients with a focus on front-end consulting.

RPS employs 5,000 people who will swell Tetra Tech’s overall staff numbers to 26,000.

Dan Batrack, Tetra Tech Chairman and CEO said: “The RPS Group advances our long-term strategy to enhance our position as the premier global high-end consulting and engineering firm focused on water, environment, sustainable infrastructure, and energy transformation.”

John Douglas, Chief Executive of RPS added: “Joining Tetra Tech represents a highly attractive combination of two leading companies in the industry for our clients and provides an ideal opportunity for our people to realize RPS’ goals to meaningfully contribute to the world’s most challenging problems in energy transformation and water management around the world.

“I am very excited about the future of our combined company and the benefits for our people and clients. By joining together, Tetra Tech provides RPS and its employees with a strong and culturally aligned platform that creates a pre-eminent consulting and engineering firm addressing climate change and all its effects.”

WSP has not given-up in its bid to buy RPS and is planning a further announcement while urging  RPS shareholders to take no action in terms of rubber-stamping the Tetra Tech deal.