Mr Woodhead said his schools would set the "highest standards".

Cognita now owns Duncombe School, in Hertford and means company, created in October 2004, runs 20 UK schools.

Its latest purchase is Quinton House, a private school in Northamptonshire.

Cognita bought the Asquith Court group of schools in the south east last month and Hydesville Tower School in Walsall last week.

Rees Withers, chief executive of Cognita, told BBC News: "I think Duncombe School is very good in many respects.

"It is a successful school in an attractive location and an area that is going to grow."

Mr Woodhead, the first Chief Inspector of Schools from 1994 to 2000, said: "It is Cognita's aim to enhance further the quality of the children's education and promote high teaching standards without compromising the ethos of the school."

Cognita is backed by Englefield Capital, a private equity firm with more than �500m of funds for buyout and development.