“This year? Next year? Sometime? Never?” These lines from the old children’s rhyme When Will I Marry? could well be applied to the prospects for Britain’s exit from the European Union. Political realities are becoming ever more complex and the final outcome is still uncertain.

The most immediate battle facing Theresa May’s government is securing comprehensive parliamentary approval for its “great repeal bill”, which transfers all EU laws and regulations into UK law. Although the House of Commons this week gave assent to the first stage of this unprecedentedly complex and detailed legislation it still faces serious challenges in the final stages of the bill’s approval.

Not only have all the opposition parties signalled their rejection of key clauses, but leading Conservative MPs have excoriated provisions that would allow the government to tamper with existing EU laws without effective parliamentary scrutiny. Unless the Tory government can force the bill through all stages of parliament in the months ahead, the final stages of UK withdrawal risk being dangerously chaotic.

Even some of Theresa May’s colleagues admit that missing the March 2019 deadline would risk a ‘cliff edge’ exit

But there are, anyway, growing doubts about whether the negotiations between the UK and the EU will be finalised when time does run out on 31 March 2019. (Allowing time for all EU states to approve any agreement means the negotiating door shuts at the end of next year.) This might well have inspired the recent resignation of the former Tory trade minister Mark Price. It would be unsurprising if others followed.

Michel Barnier – the chief EU Brexit negotiator – has left his British counterpart, David Davis, in no doubt that, unless sufficient progress has been made on the first stage of the talks, he will not recommend to the 27 EU states that negotiations begin on the second stage covering a future trade deal.

Barnier has been equally clear about why he is not ready to enter talks on a future trade deal: Westminster has yet to engage seriously with the first-stage issues, including guarantees of the future mutual rights of EU citizens. Westminster must also show how the special arrangements on free movement of people, goods and services will be guaranteed when the UK leaves the EU. Finally the UK must settle all the financial obligations it entered into as an EU member state – something it is reluctant to do.

And if talks on the crucial second stage on future EU-UK trade relations do not start until early next year, it becomes difficult to see how they could finish before the March 2019 deadline. Even some of May’s ministerial colleagues admit this would risk a potentially disastrous “cliff edge” exit, leaving industry, business and finance in chaos if the negotiating door finally closes without any trade agreement.

There is one possible way to buy more time. The UK could ask for an extension of the article 50 grace period, by requesting its extension for perhaps another year or so. This would be humiliating and it would need the unanimous agreement of all 27 EU governments. But, concerned that the chaos in the UK economy might spread throughout the EU economy, agreement on an extension of the article 50 timetable might be conceded.

This could create another hiatus, however. If the article 50 phase were to extend to, say, summer 2020, then the start of a subsequent “transitional phase” – to allow time for British industry and business to adjust to Brexit – would also have to be extended. At present it is assumed that this transition period would have to last at least two years after Brexit.

Jeremy Corbyn at the Quadrant shopping in Coatbridge, Scotland. Photograph: Jeff J Mitchell/Getty Images

But even this does not take into account the growing pressure from British business and finance for a transition period of more than two years. The Confederation of British Industry has even floated the idea of “an indefinite transition period”. But any extension of the transition could take the UK well past the time when a UK general election is due to be held. During the transition the UK would be subject to all the responsibilities of full EU membership, including financial contributions, but it would no longer have any voice or any vote in EU decision-making.

It is not hard to imagine an explosive domestic political reaction. The 48% who voted remain would be outraged at retaining all EU obligations but losing any share in EU decision-making. The old slogan “No taxation without representation” may well become very popular. And such a deal (even if agreed by the EU and only temporary) has already been denounced as rank “treachery” by the hard right, Eurosceptic, leave-supporting wing of the Tory party.

Before any of this has come to pass, the prime minister could yet be forced to resign by a mixture of internal unrest in the Conservative party and the growing unpopularity of her government on a wide range of issues well beyond Brexit. If her departure is either triggered by or otherwise leads to a vote of no confidence in the government, an early general election may be unavoidable.

An election would probably result in a Labour government led by Jeremy Corbyn, opinion poll trends suggest. Labour’s priority in government would be to drive a new anti-austerity growth and employment strategy. Some in the Labour movement are already urging Corbyn to reach out to the rest of the EU with a policy for cooperation and reform. There are clear signs that the political mood in the EU is changing: austerity is being rejected and there is a new emphasis on the need for a Europe-wide economic stimulus programme to boost investment recovery.

Recent reports that EU governments want a significant change in tax strategy, designed to hit grossly undertaxed global corporations (taxing them on turnover, not on easily manipulated profit figures), point in the same direction – as do plans to unveil a strengthened “pillar of social rights”.

For Labour’s anti-austerity recovery strategy to succeed, it will be vital that the UK and the rest of the EU march in step with each other, with new policies for sustainable growth, greater social equality and improved workers’ rights. This, of course, will mean radical reforms both in Britain and throughout the union. This would be a constructive way to demonstrate that the European commission president Jean-Claude Juncker’s prophecy in Strasbourg – that Britain will “soon regret” Brexit – need never come to pass.

It is hard to see a better way of pursuing this than for a Labour government to make it clear that it will fight any new election rejecting the Conservative government’s chaotic flirtation with disaster, and seek to retain (or recover) EU membership for the UK. However corrosive the UK’s tryst with Brexit has so far proved, a Labour determination to reverse it would be good news for Britain and for the European Union as a whole.

• John Palmer is the Guardian’s former Europe editor