Almost 46 years ago, on 21 October 1971, the Conservative foreign secretary Sir Alec Douglas-Home rose in the House of Commons to open what had become known in the country as “the great debate”. The matter MPs were about to discuss was, Douglas-Home told them, “momentous”. The motion to be voted upon stated that “this house approves Her Majesty’s government’s decision of principle to join the European Communities on the basis of arrangements which have been negotiated”.

The country was deeply divided over whether the UK should throw in its lot with the Europeans, and the Labour party in particular was profoundly split. After more than 300 hours of debate, Tory prime minister Edward Heath was helped across the line by pro-EEC Labour MPs, including a young David Owen, and the 1972 European communities bill passed into law.

When the UK officially joined – on 1 January 1973 – the Guardian raised doubts about the new union, noting that British people tended to blame others wherever they could. “One temptation should be avoided – to seek, month after month, to prove that membership of the community has created all Britain’s ills,” it said. “Above all, we should avoid creating a new, semi-permanent rift in British society, between pro- and anti-Europeans.”

It was a vain hope. On Monday night, Tory and Labour MPs, their parties riven by disagreements over Europe, will vote for the first time in another “great debate” – this time on the European Union (withdrawal) bill, which will strike from the statute book the legislation that was passed in 1972.

If joining was divisive, the process of leaving after four and a half decades looks like being even more angrily contested. The current “Brexit parliament” will debate little else, so complex and contentious are the issues to be dealt with.

One senior Labour source at the heart of cross-party attempts to amend the withdrawal bill said the legislation would be “eaten alive” in both the Commons and Lords over the next few months by those who oppose a hard Brexit. The vast majority of MPs agree that the decision to leave the EU is pretty much irreversible. But on how we leave and when, the disagreements run so deep, and are so numerous, that the passage of the bill towards the statute book will be a deeply troubled one, if indeed it completes the journey at all.

With Theresa May lacking a Commons majority, and dependent on the 10 Democratic Unionists to stay in Downing Street, each vote is potentially perilous. Monday will in all likelihood be the least problematic day for May, as rebels rarely break cover at a bill’s second reading. They keep their powder dry and light the fuses later. Labour will, however, make things as awkward as it can from the start, whipping its MPs to vote against a second reading, along with the other opposition parties, because it says the bill grants ministers excessive levels of executive authority that would allow them to bypass MPs – the so called Henry VIII powers – and is deeply flawed in many other respects.

Plenty of Tories are just as unhappy and have said so, but most will bide their time before defying the whips. Any Conservatives who do rebel tomorrow (the Europhile Kenneth Clarke may abstain) will be more than cancelled out by hard-Brexit Labour rebels like Kate Hoey, who will defy their own leader and vote for the bill to proceed.

Europhile Tory MP Kenneth Clarke may abstain on the EU withdrawal bill. Photograph: Peter Macdiarmid/Getty Images

The real trouble will start this autumn when the bill comes back to parliament after the party conference season, for detailed scrutiny. Already, cross-party alliances are forming between Tory MPs who oppose a hard Brexit and those of like mind from Labour, the Liberal Democrats, the Scottish National party and the Greens.

Plenty of pro-EU Tories want to keep open the option of staying in the single market, as does Labour, and want a binding vote on any eventual deal, as does Labour.

One Tory veteran of the Maastricht debates of the early 1990s said the Brexit parliament would see the Conservatives split even more profoundly than they were then. “Maastricht damaged us terribly,” said the MP. “But I think the following months will be far, far worse.”

Secret meetings to plot tactics have already been held between Tory and Labour MPs on both sides of the argument and between Keir Starmer, Labour’s Brexit spokesman, Lib Dem leader Vince Cable, and the SNP. On the Tory side, party discipline is fast disintegrating, as was clear on Thursday when 40 Conservative MPs wrote a letter calling for the hardest of hard Brexits in defiance of official government policy. On the same day, in the Commons chamber, Tory MPs including Clarke openly praised a speech by Starmer as “brilliant”, while former Tory attorney general Dominic Grieve described the bill before them – brought forward by his own party – as an “astonishing monstrosity”.

Labour and pro-EU Tory MPs are already working on how to table amendments on the single market and other issues through select committees or other backbench groups so that they can be presented as having all-party backing and attract maximum levels of cross-party support.

The intra-party splits and rebellions are already out in the open. The large anti-hard-Brexit majority in the House of Lords could also give fair wind to such amendments. Writing for the Observer, the Labour peer Lord Adonis says he believes that by early next year the Lords will vote for an amendment calling for a referendum on the final deal. “It is vital this is not conceived as a rerun of last year’s poll, but rather a referendum on May’s deal. And it is essential that, come the referendum, there is a credible and optimistic alternative to accepting withdrawal.”

Labour, he thinks, will eventually back such a move. The SNP’s Brexit spokesman Stephen Gethins, writing on theguardian.com, says there is emerging “common ground” in the Commons between the parties on everything from the rights of EU citizens to preserving the UK’s place in the single market. The soft-Brexit Tory Europhile Anna Soubry has no qualms in saying she will happily back any changes to the bill if she believes they would have the effect of keeping the UK in the single market and customs union. In a speech in the Commons on Friday Clarke, now father of the house, admitted that the main parties were hugely split on Brexit, as they had been for decades. He appealed for unity and a common sense approach.

“The public are not idiots,” he said, “they know that both parties are completely and fundamentally divided on many of these issues, with extreme positions on both sides represented in the cabinet and shadow cabinet, let alone the back benches.

“Let us therefore resolve this matter. Let us make sure this bill does not make it impossible to stay in the single market and customs union and let us have a grown-up debate on the whole practical problem we face, and produce a much better act of parliament than the bill represents at the moment.”

Clarke, who was a junior whip in the Heath government that passed the 1972 act, is clearly still hoping that MPs could come together on Europe in the national interest as the UK leaves the EU. But more than any other MP in the Commons today, he will know how difficult it will be to achieve that kind of unity in a divided house.

Liberal Democrat leader Vince Cable with Pro-EU demonstrators. Photograph: Niklas Hallen/AFP/Getty Images

The Brexit tribes







No Brexiters

They think Brexit will be a disaster for the country and hope it’s not too late to think again. The Lib Dems, led by Vince Cable, are the outriders for this group. The Green party (only one MP) also backs a second referendum. The SNP wants to find any way it can to keep Scotland in the EU. A few Labour MPs, including Chris Leslie, are also moving towards the idea of a second referendum. On the Tory side, Ken Clarke opposes Brexit full stop and voted against triggering article 50, though he does not approve of referendums.

Leading lights



Vince Cable (Lib Dem)



Ken Clarke (Conservative)



Chris Leslie (Labour)



Stephen Gethins (SNP)



House of Commons presence

50+ MPs

Marshmallow Brexiters

As soft as it’s possible to be, this group concedes that Brexit probably will and probably should happen. But these MPs are desperate for the UK to retain full membership of the single market and customs union – and not just in a transitional phase. In their ranks are 50 Labour MPs (49 voted for a strong pro-single-market amendment tabled by Chuka Umunna in June) and as many as a dozen Tories. Plaid Cymru MPs are also sympathetic. They don’t really want to leave, but they don’t want to defy the referendum result.

Leading lights



Nicky Morgan, Anna Soubry (Conservative)



Chuka Umunna, Heidi Alexander (Labour)



Hywel Williams (Plaid Cymru)



House of Commons presence

60+ MPs

Softly, softly Brexiters

To turn Macbeth’s reasoning on its head, this group of MPs believe that “If it were done … then ’twere well it were done very, very slowly”. And they will be a major force in parliament. Labour’s new position, supported by most of its 262 MPs, is to back single market and customs union membership during a transitional period of up to four years, keeping the option of remaining inside both if reforms to free movement can be negotiated with Brussels. Plenty of Tory MPs would sign up to this as well. In the Brexit parliament this is the mainstream opposition to Theresa May.

Leading lights



Keir Starmer (Labour)



Crispin Blunt (Conservative)



House of Commons presence:



250+ MPs

Resolute leavers

The “full steam ahead” gang. For these MPs, and for May, Brexit demands leaving the single market and customs union completely and for good from March 2019. The UK would seek a bespoke arrangement with the EU for any transition period. The 10 DUP MPs who are propping up the May administration have hopped on board for this ride, though they strongly oppose a hard border between Northern Ireland and the Republic.

Leading lights



Graham Brady (chair of the 1922 committee of Tory backbenchers)



Caroline Flint (Labour)



House of Commons presence

200+ MPs

Labour MP Caroline Flint, a resolute leaver. Photograph: Leon Neal/AFP/Getty Images

Brexit crusaders

No turning back. No compromise. This group of hardliners will not accept anything less than pure and total Brexit, with all legal and financial ties cut from March 2019 onwards. If that means no deal on Brexit with the EU, and trading on World Trade Organisation rules, then so be it. Freedom from EU control is more important than any damage to the economy, which these MPs think would be temporary.

Leading lights



John Redwood and Jacob Rees-Mogg (Conservative)



Kate Hoey (Labour)



House of Commons presence

20-30 MPs