Grammar schools are "stuffed full of middle-class kids" and do not improve social mobility, according to the head of Ofsted after a bid for a new grammar school annexe in Kent was rejected.

In highly critical comments, the man in charge of improving standards in the UK, Michael Wilshaw, said grammar schools did not work and they should not be allowed to expand.

He said more selection would not improve the UK's standards after a report comparing international standards saw the UK faring badly.

His intervention comes only days after education secretary Michael Gove blocked a plan for a new grammar annexe in Kent.

He said he did so because the proposals made by two schools to act as a sponsor would breach the law.

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

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

He said while grammars might help with 10% of the school population, in areas where they still existed like Kent "everyone else does really badly".

In an interview, he urged the government to resist calls for a return to selection.

"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."

His comments will re-open the debate about selection and come as Kent County Council prepares to unveil measures to try and encourage more children from disadvantaged backgrounds to take the test.

The council is in the process of putting together a series of initiatives aimed at helping schools identify brighter pupils from poorer backgrounds to apply to grammar school.

Mr Wilshaw said the focus should be on improving the country's current schools.

Meanwhile, UKIP leader Nigel Farage exploited the Conservatives' discomfort over the rejection of a new grammar school in Sevenoaks, labelling it as "a disgusting betrayal of social mobility".

He said: "Over two and half thousand parents signed a petition demanding a new grammar school in Sevenoaks.

"By doing this the Conservatives have exposed themselves as being a party that stands against academic selection and school choice. It is a disgusting betrayal of social mobility in Britain."

Mr Farage added: "UKIP has long supported grammar schools so that those from the poorest backgrounds can get on and reach their full potential.

"We will continue to relentlessly make the case for grammar schools to be created right across the country. The demand is clearly there from parents."

What do you think? Do you agree with grammar schools or are they an outdated institution?