What are grammar schools and why are they so controversial?

As debate rages about Theresa May's plans for a new batch of grammar schools, we take a look at their history and see how exactly they work.

• MORE: PM unveils plans for new generation of grammar schools - what do you think?

Q What is a grammar school?

A grammar school is a state secondary school that selects pupils by an exam taken by 11-year-olds, known as the 11-plus.

There are currently 163 grammar schools in England, out of more than 3,000 state secondary schools.

In the traditional grammar school education system pupils who pass the 11-plus while at primary school can attend the local grammar, while those who fail it go to the local secondary modern school.

The comprehensive system has been favoured in the UK, where pupils of all abilities and aptitudes are taught together.

Q When were they first introduced?

Grammar schools have existed in some form for centuries, although the modern grammar concept dates back to the Education Act 1944, which made secondary education free for over-14s.

At the time schools were reorganised into two main types: grammar schools and secondary modern schools, the latter of which was aimed at children who were going into trades.

The system divided pupils into those heading to university and those deemed more suitable for lower paid jobs.

Q When did the system change?

Critics had long argued that the selective education system reinforced class division and middle-class privilege. In the mid-1960s the Government ordered local authorities to start phasing out grammar schools and secondary moderns, and replace them with a comprehensive system.

It triggered a dramatic decline in the number of grammar schools in England and Wales. In 1965 there were around 1,300, but the figure had dropped to 163 by the mid-1980s.

No new grammar schools have been built in England since 1968.

In 1998, Labour's School Standards and Framework Act banned the establishment of any new all-selective schools.

A handful of counties and local authorities in England have kept largely selective schools systems, including Kent, Medway, Buckinghamshire and Lincolnshire. Only a handful of state grammar schools exist in the Black Country and Staffordshire.

Q How are grammar school pupils selected today?

Pupils who want to attend grammar school must take the 11-plus in their last year of primary school. The entrance exams consist of questions that include: numerical reasoning (maths), verbal reasoning, English comprehension, punctuation and grammar, non-verbal reasoning and creative writing. Critics complain about the nature of the test, as the types of questions featured are not usually covered in regular state primaries.

Some grammar schools admit successful students by ranked order, taking the best performers, while in other areas pupils who pass the test are ranked by admission criteria.

This can include the distance they live from the school or whether they already have a sibling there.

Q Who is against grammar schools?

Labour opposes the creation of more grammar schools, arguing that instead of improving equality, they make it worse. The party says that the Government should focus on improving other aspects of the state education system.

The Lib Dems would maintain the status quo, not opening any more grammars or closing any existing ones.

Ofsted chief Sir Michael Wilshaw is another opponent. He says plans to change the educations system could halt the dramatic improvements in the country's education standards.

Some Tory MPs have also expressed concerns over grammar school expansion. They include Dr Sarah Wollaston, who said such a policy could undermine the Prime Minister's vow to reduce social mobility.

Q Who says they are a good idea?

Under David Cameron the Government's support for grammar schools was lukewarm, but that has now changed under Theresa May. Some opinion polls suggest that public support for new grammar schools has risen in recent years.

Q What are the advantages of grammar schools?

Supporters argue that grammar schools undermine privilege by helping brighter children from lower income homes. Grammars also traditionally boast strong exam results, because teachers are dealing with only the most able pupils.

Q What are the disadvantages of grammar schools?

Critics of grammar schools say they are divisive, with wealthier children more likely to receive a place because their parents can pay for tutoring. Grammars are also seen as being bad for social mobility by selecting children at such a young age. Just three per cent of grammar school pupils come from families on benefits or earning less than £16,000 a year.

Q What happens now?

Theresa May's proposals will go before the Commons in what is sure to be a heated debate.