Tim Farron is Leader of the Liberal Democrats and is MP for Westmorland & Lonsdale.

There was a Conservative Education Secretary who closed 403 grammar schools. Her name was Margaret Thatcher.  There was another Conservative Education Secretary who said: “The evidence is now incontrovertibly clear that a rigorous academic education does not need to be the preserve of the few.” That was Nicky Morgan, in charge of British education until just two months ago.

I am grateful for this opportunity to write for Conservative Home, and as a polite guest have no intention of insulting my hosts. Rather than point-scoring, I’m here to offer candid advice on grammar schools and explore whether we can work together to improve education for all. Because, whatever side you are on, we can agree that the debate about and choice between selective and non-selective education is massively important, and could help shape society for generations.

Conservative MPs in their constituencies this weekend will have heard concerns about the Prime Minister’s plans to bring back not merely a few symbolic grammars, but potentially one in every town. Older MPs will recall how unpopular a system of segregation was even among Conservatives – reflecting the fact that far more parents had children who failed the 11 Plus than passed it, and that, bluntly, they had votes too.

I do not impugn the motives of Conservatives anxious to raise standards. From the Centre-Right to Centre-Left there is consensus that our number one domestic challenge is to increase social mobility. Graham Brady has said we should not allow the seven per cent fortunate enough to attend private school to dominate public life, and I agree.

But if our shared goal is to provide opportunity for everyone regardless of background, I humbly suggest that grammars are not the answer. No Conservative MP who believes in meritocracy can seriously defend a system that, for all the soothing words, effectively brands many 11-year-old children failures.

By 11, only three-quarters of the poorest children reach the Government’s expected level, compared to 97 per cent of the wealthiest kids. Some argue that successful Local Education Authorities tend to contain grammars. The evidence is at best mixed. Selective areas are often richer, which boosts results. They also draw bright children from nearby areas. Only the richest five per cent of kids do better in selective areas. Kids from the least wealthy half of households actually do worse.

But here is the killer stat: the Sutton Trust found that less than three per cent of grammar kids were on free lunches, compared with 20 per cent across the country.

Theresa May addresses this by talking of “inclusive grammar schools”. But this is a contradiction: grammars exclude. Some complicated two-tier entry taking into account socio-economic background will have MPs post-bags brimming with constituents complaining that their child was denied a place despite gaining a higher mark than another child.

And however “fairly” you try to select, you won’t escape the fundamental unfairness of deciding ability at the age of 11.  Streaming within schools works because it allows flexibility as children flourish at different stages and in different subjects. Grammars stream people for entire school careers, and beyond.

My frustration is that there are so many areas in education where we could make cross-party, positive reforms. If we agree that the chance of birth still plays far too big a role in determining outcomes, look at the difference we have made already – and we can now build on.

In coalition, Conservative MPs went into the lobbies, and voted for Liberal Democrat measures such as free early years learning, pupil premium, free school dinners and national apprenticeship scheme. These targeted resources at those who needed them most – progressive reforms that increased life chances and will ultimately benefit the economy and society. Crucially, in coalition we intervened where you make the most difference in boosting life chances: in the early years.

My plea to you is that rather than introduce policies that entrench division, let us explore together fresh ways to increase opportunities for all, regardless of background. Let us be generous with resources and imaginative with ideas.

But even if you reject my educational arguments, please consider the politics. The Conservatives have a majority, effectively, of 12. This parliament will be dominated by Brexit in all its guises, and even a government with a majority of 112 would find this a monumental challenge.

A growing number of Conservative MPs are expressing grave doubts about a sudden U-turn that abandons a main plank of David Cameron’s modernising agenda and, more importantly, was not in the Conservative manifesto. As a Liberal Democrat, I speak from experience when I say that letting people down on education can be very, very damaging.

It would only take half a dozen Conservative MPs to vote against this proposal, and suddenly Theresa May’s premiership could look severely weakened, which would also make it hard to conduct with authority the vital international negotiations that await. Remember how Gordon Brown, who also became Prime Minister by succession, went from being master of the universe to dead man walking almost overnight – and he wasn’t attempting anything on the scale of Brexit or the entire overhaul of our school system, let alone both. Do you really want a war with teachers, -with arguments over structures distracting them from teaching our children?

So as MPs return to Westminster this morning, I politely suggest they deliver a simple message to the Conservative Chief Whip: the national return of grammar schools does not command the support of the country, and will not command the support of either House.

Whatever your views on Brexit, Britain is more sharply divided than in a generation. Any One Nation Conservative will want to ponder very hard before supporting arguably the most divisive policy in British education. Now, more than ever, politicians across all parties should work to keep Britain open, tolerant and united.