The recommended offer for energy and heating specialist Eaga values the firm at £306.5m.
At a stroke the deal makes Carillion the largest player in the fast growing local authority and housing association energy sector where billions of pounds are expected to be invested over coming years.
Carillion said the takeover would secure its position in the support services market, creating a business with a combined support services revenue of nearly £3bn.
Eaga runs large national projects such as the Government’s Warm Front home insulation scheme and turned over £762m last year and employs around 4,000 staff.
The firm has recently made big inroads into fitting solar panels to local authority buildings and homes.
Carillion will bring its financial muscle to the business helping to develop large-scale low carbon energy projects for the public sector.
Eaga has already secured contracts with a number of social landlords, giving it access to approximately 120,000 social landlord properties, with a further 200,000 properties in the pipeline.
Philip Rogerson, chairman of Carillion, said: “The acquisition brings together two complementary companies, enhancing Carillion’s position as one of the UK’s leading support services companies.
“Carillion has identified the low carbon market as a strategic area of growth and the acquisition of Eaga will create a scalable platform to build the UK’s largest independent energy services provider.
“This will also extend Carillion’s capability to provide integrated support services solutions for its existing customers, for whom energy services are an increasingly important requirement.”
Charles Berry, Chairman of Eaga, said: “The offer received from Carillion has come at an interesting time in Eaga’s development, as our markets are changing rapidly.
“While there are exciting future prospects, we believe these are potentially better accessed as part of a larger group.
“Carillion offers our unique business the opportunity to grow in a strong home, it offers our Partners the prospect of delivering that growth potential, while our shareholders receive a significant cash premium and a partial share alternative which allows them to participate in the Enlarged Group’s future potential.”