Promanex operates across the UK and in Ireland with a workforce of over 800 maintenance staff and gives Costain the ability to offer full lifecycle services to customers in these growth markets.

The £56m turnover Warwick-based firm carries out facilities management, installation, repair and maintenance and general asset management.

Costain funded the deal from its cash pot which, together with management retention payments, cost £16.4m.

In addition, the business is being acquired with normalised net debt of £2.4m.

Promanex has built up a strong customer base in recent years and will give Costain access to major blue chip client like Conoco Phillips, Scottish and Southern Energy, Siemens and Total.

Andrew Wyllie, chief executive of Costain, said: “We are delighted to have completed the acquisition of Promanex, which represents a further important step in the implementation of our strategy of broadening our existing front-end consultancy and care and maintenance operations.

“The acquisition will significantly enhance our existing ability to provide operations and maintenance support services to our major customers and reinforces Costain’s position as a leading Tier One engineering solutions provider.”

Promanex offers a range of services in the management, operation, maintenance and repair of customers’ sites and plant assets, including hard and soft facilities management.

Around 75% of Promanex’s revenues come from long term contracts which helped to generate an operating profit of £1.8m last year.

The contractor will be integrated into Costain’s Infrastructure division with the existing management team staying on to ensure a successful integration and continued growth in the business.