Grammar schools "don't work" at increasing social mobility and are "stuffed full" of middle-class children, Sir Michael Wilshaw has said.

In an interview with the Observer, the Ofsted chief inspector also said summer holidays were "too long" and admitted that he had concerns about the number of Romanian and Bulgarian children who might come to British schools after restrictions on the two European Union countries expire at the end of the year.

"Grammar schools are stuffed full of middle-class kids. A tiny percentage are on free school meals: 3%. That is a nonsense," he said.

"Anyone who thinks grammar schools are going to increase social mobility needs to look at those figures. I don't think they work.

"The fact of the matter is that there will be calls for a return to the grammar school system. Well, look what is happening at the moment.

"Northern Ireland has a selective system and they did worse than us in the (international comparison) table. The grammar schools might do well with 10% of the school population, but everyone else does really badly. What we have to do is make sure all schools do well in the areas in which they are located."

Sir Michael's criticism of state grammar schools in England comes after the rejection of an attempt to expand the provision of selective education in Kent.

Weald of Kent Grammar School and Invicta Grammar School both put in rival bids to run an annexe in Sevenoaks to address a shortage of selective places.

Both were rejected by the Department for Education because neither complied with the law, which allows expansion but not new schools.

In a wide-ranging interview Sir Michael indicated he would like to see changes to the school calendar when asked if teachers had too many holidays, saying: "I think the six-week holiday is too long."

He acknowledged that the Government had a "big job" in ensuring England had enough quality teachers in the system to cope with a potential influx of Romanian and Bulgarian children when the restrictions on people from the two countries working in the UK are lifted.

"Obviously I have got concerns about that," he said. "If we get huge numbers of children from overseas and from eastern Europe coming in, we have to have enough teachers to teach them and resources available in schools."