The use of privately contracted school inspectors is on the rise, despite concerns about their "inconsistent" performance.
Additional inspectors (AIs), employed by private firms which carry out inspections for Ofsted, have cost #163;126 million over the last three years, figures released to The TES have revealed.
They led 78 per cent of the state schools inspections carried out in 200910, up from 76 per cent the previous year.
Representatives from the three independent service providers (ISPs) - CfBT, Tribal and Serco - told Wednesday's hearing of the Commons education select committee, which is scrutinising the role and performance of Ofsted, that some inspectors had not taught in a classroom for more than 20 years.
Several submissions to the committee raised concerns about the quality of AIs' inspections compared to those carried out by Her Majesty's Inspectors (HMIs), who are employed directly by Ofsted.
The ISPs insist AIs' work is carefully monitored so there is "little discernible difference" between them and HMIs - and Ofsted admitted to The TES that it could not afford to employ the extra 150 HMIs which would be needed if they were to lead all of its inspections.
A written submission from teaching union NASUWT said it "remains concerned that in too many instances ... inspectors' knowledge, experience and understanding of key elements of the areas they are seeking to inspect are inadequate or insufficiently up to date".
Anastasia de Waal, deputy director for research at think-tank Civitas, told an earlier committee hearing that schools "prefer" being inspected by HMIs as "they feel they are getting additional advice".