Inspectors have warned the Oasis multi-academy trust (MAT) that a “legacy of weak challenge” meant too many of its schools were not “improving quickly enough”.

Ofsted made the comments after a “focused inspection” of the academy chain.

The watchdog said more needed to be done to improve the progress of students, particularly boys from poorer backgrounds, who were lagging behind their peers nationally.  

And while some schools were shown to be closing the attainment gap between disadvantaged and more affluent students, across the chain as a whole the issue remained a “concern”.

The findings are likely to prompt further criticism of the government’s push to convert more schools into academies. On Monday, prime minister David Cameron said even schools judged “good” by Ofsted could be forced to become academies.

Ofsted gave its verdict on Oasis, which has 44 academies, after sending full inspection teams into eight schools and undertaking monitoring visits of two more. It also carried out telephone interviews with a further 20 academy principals.

In its letter, the inspectorate says: “The trust’s academies have a variable record of improvement. While some have improved or sustained effective performance, too many academies have not improved quickly enough. A legacy of weak challenge and insufficiently systematic or rigorous improvement work has resulted in slow or little improvement for nearly half the academies.”  

Of the eight full inspections carried out in March, four schools were rated “good”, three were judged to “require improvement” and one was placed into special measures.

Oasis said it “welcomed” Ofsted’s decision to inspect its schools as an MAT, but the chain raised “serious concerns” about the methodology the watchdog used in its report.

“At the same time that our national office was inspected, 10 of our academies were subject to batch inspection,” Oasis said in a statement. “The results of 90 per cent of these inspections were incredibly positive. The secretary of state has explicitly stated that MAT inspections should be rooted in the performance of individual academies during Ofsted inspections. We have serious questions as to whether this report truly reflects this.”