Education watchdog Ofsted has grown "enormously" and faces the risk of becoming "unwieldy", according to a report published today.

The Commons children, schools and families committee report into school accountability also warns against government plans to rate schools with a single grade on a US-style report card.

"We now have an accountability framework which is far too complex and which stigmatises and undermines struggling schools," committee chairman Barry Sheerman said. 

"What schools need now is a period of stability and a chance for their own efforts to improve performance to bear fruit."

The report questions whether Ofsted's work, inspecting not just schools but colleges and children's services, is "sustainable" in the long term.

It states: "Both Ofsted and the government should be alert to any sign that the growth of Ofsted's responsibilities is causing it to become unwieldy or unco-ordinated."

Of the proposed new 'report card', the committee says: "A report card can never be a full account of a school's performance, yet the inclusion of an overall score would suggest that it was."

Research carried out last year by the Department for Children, Schools and Families (DCSF) suggested parents opposed the plan, which was proposed by schools secretary Ed Balls in 2008.

Many were worried it would "stigmatise" poorly performing schools, and result in lower esteem among pupils and teachers.

Under the system, state schools will be ranked on a number of measures, including behaviour, attendance and take-up of sport, as well as academic performance - and given a final overall grade.

John Dunford, general secretary of the Association of School and College Leaders (ASCL), said the government's obsession with accountability had created a false sense of crisis in schools.

"Many schools, and by association the pupils, have been unfairly damaged by the preoccupation with misleading league tables and with 'naming and shaming' those deemed not to be doing well enough," Dunford said.

"Too much effort is spent on deciding which schools belong in various categories of failure, and which schools should be awarded prizes and plaudits. Sometimes the same schools appear in both categories."

John Bangs, head of education at the National Union of Teachers, described the report as "the final nail in the coffin for the government's current overwhelming school evaluation arrangements".

A spokesman for Ofsted said recent changes mean inspectors now spend more time in classrooms observing teachers and pupils.

He added: "Of course, test and examination results are important and it would be wrong to ignore overall results. This is what parents and employers look for.

"While pupils' learning and progress remain at the heart of inspectors' professional judgments, it is unusual for schools to get good results without good teaching. Our approach allows us to recognise good work in all schools, especially those in the most challenging circumstances."