The Cheshire-based group, which employs 15,000 people and operates in 40 countries, has received the investment from a consortium led by Searchlight Capital Partners and Ares Management Corporation funds.

Ares has also committed an incremental £300m debt facility, bringing total available debt facilities provided by Ares to £1.4bn.

The equity investment deal will unleash even more ambitious expansion in the years ahead.

RSK aims to quadruple its turnover from this year’s £1.2bn to more than £5bn in 2030, with an extraordinary target of doubling its family of environmental and engineering businesses from 200 to 400.

Founded in 1989 by its chief executive Alan Ryder, RSK’s family of firms together offer end-to-end, bespoke environmental consulting and engineering solutions to clients across the water, energy, construction and infrastructure sectors.

Ryder said: “RSK is very pleased to welcome a new group of investors and further strengthen our relationship with Ares.

“This strategic partnership marks a pivotal moment for RSK as we take another step towards our long-term vision of becoming the world’s largest and most trusted provider of sustainable solutions.

“Searchlight and Ares’ expertise in our industry, combined with their track record of supporting growth initiatives, will be instrumental as we execute on our M&A strategy and expand our service offerings.

“After three decades, RSK remains committed to delivering environmental and engineering services to help government and business clients around the world achieve a more sustainable future. The opportunities for RSK increase at pace.”