After a cross-party alliance formed to fight Theresa May’s plans to expand the number of selective schools in the UK, the debate over grammar schools’ effectiveness raises its head once again.

In an unprecedented move, former education secretary Nicky Morgan, Liberal Democrat former deputy prime minister Nick Clegg, and former shadow education secretary Lucy Powell have joined forced against the proposal.

In a joint article in the Observer, the trio said more grammar schools was "not the answer" to improving social mobility.

They said: "Politics is often defined by what people disagree on. However, some issues are above party politics and it’s time that tackling social mobility became one of them."

In the article, the group say social mobility has in the past been "too narrowly defined", and only results in a tiny portion of disadvantaged children being elevated.

They argue that adequate resources must be afforded to all schools, rather than putting money into more grammar schools.

When Mrs May introduced the policy last year, she said: "For too long we have tolerated a system that contains an arbitrary rule preventing selective schools from being established – sacrificing children's potential because of dogma and ideology. 

"The truth is that we already have selection in our school system – and it's selection by house price, selection by wealth. That is simply unfair." 

She said she wanted to make England a "country that works for everyone".

Grammar schools are selective state secondary schools that accept students by way of an examination taken at age 11, the 11-plus.

There are currently 164 in England but they have been around since the 16th Century.

The modern version dates back to the Education Act 1944 which made secondary education free.

At their peak, there were more than 1,200 grammar schools in the UK. However, in the 1950s and 60s, Labour began to campaign against them, arguing they created a greater class divide. In 1965, grammars began to be phased out and replaced with a comprehensive system.

A few counties retained the selective system including Kent, Medway, Buckinghamshire and Lincolnshire. The 11+ exam is taken in the last year of primary and is available to any pupil. However, opponents argue the test includes questions children from normal state primaries are not prepared for, giving students tutored extensively more of a chance of passing.

On top of this, almost half the children who pass the 11-plus exam fail to get a place.

It’s argued social mobility will increase by putting grammar schools in areas of deprivation, but in 2013, for example, two-thirds of pupils at grammar schools in Stoke-on-Trent and Kingston lived in a different authority area.

Less than three per cent of pupils in grammars are eligible for school meals.

Students who manage to get into grammar schools do achieve significantly better results than non-fee paying schools.

Chairman of the National Grammar Schools Association Robert McCartney said: "They have an absolutely proven track record as being excellent schools.

"Grammar schools bring together the pupils most suited to an academic education. You will have two forms doing physics, which is not even an option is some non-selective schools."

Mr McCartney, from a working class family and the youngest of eight, said the trajectory of his life was completely changed when he was accepted into a grammar school. He went on to university, became an MP, and had four children who also went to grammar schools, one of whom attended Oxford.

He said:  "If we’re talking about social mobility, the knock-on effect of one child out of eight going to a grammar school is clear. I want children in 2017 to have the same opportunities I had."

The former independent MP believes the vastly reduced number of grammars means the majority of children who can go are those who benefit from domestic advantages, such as a greater range in vocabulary and private coaching.

Mr McCartney also criticised the commonly reported percentage of pupils who rely on free school meals in grammar schools.

He said: "The proportion of students entitled to free school meals in grammar schools is three per cent. But that is across all pupils in all schools, from those taking remedial classes to those taking physics.

"The ones who have managed to get into grammar school are in a totally different category. They are in the top 15 per cent in the country; you need to compare like with like. It’s a completely false comparison."

Regardless of the academic results grammars achieve, putting money into them at a time when there is a school funding crisis has been strongly criticised.

General secretary of National Union of Teachers, Kevin Courtney, said schools are "crying out" for sufficient funding.

He said: "Instead of addressing this as an urgent priority, the Education Secretary is planning to return to a two-tier education system in which 90 per cent of children will attend what are in effect secondary-modern schools.

"The Government knows there is neither evidence nor support for a return to selective education. The NUT is quite clear that selection within academy trusts is currently unlawful and yet the Government is quietly encouraging academies to go down this route."

Tim Knox, director of the Centre for Policy Studies, doesn’t believe the introduction of more grammars will make a huge difference.

A supporter of the schools, he said because there is much choice when it comes to schools, "it’s not education apartheid anymore" and helps children from poor backgrounds with the potential to attend get a much better education.

He admits the less than three per cent is not good enough and the Government should do more to incentivise disadvantaged children to apply.

He suggest giving those potential students access to the resources, such as previous 11-plus papers, afforded to those from wealthier backgrounds.

The Government is expected to produce a white paper soon in response to a consultation on the proposals.

No grammar school in south London responded for comment for this feature.