Holyport College in Berkshire will boast Eton as its educational sponsor and 225 of the 500 pupils will be boarders.
The job has been procured through the Scape framework and Willmott Dixon will start construction this month with the school set to open in September 2014.
Work will see the company demolish some of the existing buildings and replace them with new teaching and living accommodation.
Eton College will take a lead role on the Governing Body, helping to shape the school ready for opening next year, with the school also able to use some of Eton’s facilities, particularly for sport.
Willmott Dixon managing director for the southern Home Counties John Waterman said: “This is a very exciting development for the whole free school movement and one we are delighted to be helping make happen.
“The site team are working closely with the College to ensure their requirements and needs are delivered on this unique project.
“As with all our projects, we’ll use local companies wherever possible, to ensure we use it to sustain jobs and help the local economy prosper from having such a prestigious College built in their region.
“We have a reputation as a considerate builder and will endeavour to minimise disruption for those who live, work and use the roads near the site.”
Simon Dudley, Chairman of Governors of Holyport College, said, “We are delighted that Willmott Dixon is on board and look forward to welcoming our first students at the beginning of September 2014.”