The deal for an undisclosed sum cuts the family-owned south east contractor’s ties with civil engineering on which it was founded 55 years ago.

The remaining £250m turnover construction business will get a big cash injection to now focus on residential and education building projects.

Osborne Infrastructure Limited is a leading regional civil engineering and transport infrastructure business, with longstanding relationships with key clients Network Rail, Highways England, and Transport for London.

New independent civil contractor will be led by John Dowsett as CEO and David Fison as chairman

The independent infrastructure contractor will continue to operate under the leadership of managing director John Dowsett, who will now become CEO.

David Fison, former CEO of Skanska UK and Osborne Group for six years, will return to the civils business as chairman.

Richard Sanders, Founding Partner of Sullivan Street, said: “We are excited to be acquiring a business with a fantastic reputation and which provides essential infrastructure to the nation’s transport networks.

“We look forward to supporting the management team as OIL enters its next chapter as a focused independent business.”

Dowsett said: “We are delighted to welcome the investment and support from Sullivan Street which will help us to achieve the growth ambitions set out within our business plan.”

Andy Steele, Chief Executive of Osborne added: “Having secured the very best sponsor for Osborne Infrastructure Ltd, we now have an amazing opportunity to prioritise our investments and focus our energy and resources into becoming market leaders in the residential and education sectors.

“This move provides us greater clarity of purpose and with our in-house expertise we can add value into every stage of the property lifecycle.

“Through offering end-to-end land acquisition through operation and property maintenance, we will provide sustainable, intelligently designed spaces that improve people’s lives.”