The first line of defence against a no-deal Brexit used to be the fact that it would destroy millions of livelihoods, torpedo the economy and cripple our national infrastructure. Accordingly, the second was that no rational prime minister would ever seriously countenance it. Both lines have now fallen. That self-proclaimed bastion of dependable governance, the Tory establishment, has given itself up to fanaticism, and even MPs known in a previous life for their centrist mildness have now committed to support a voluntary economic crisis for the good of the party.

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We must therefore concede that the risk of no deal has appreciated. Both Boris Johnson and Jeremy Hunt have pledged to implement it if they cannot renegotiate the withdrawal deal, while the EU has staked its credibility and cohesion on not renegotiating it. Brussels did not cave in to Theresa May and will certainly not submit to her successor.

That leaves us with the final and most important line of defence. Under our improvised constitution it is the only real guarantee against tyranny. Brexiters once considered it so vital to their cause that they placed it at the centre of their referendum campaign. This is the sovereignty of parliament. If the prime minister is out of control, MPs will take it back.

Sceptics can be forgiven for doubting parliament’s resolve. MPs have been offered numerous opportunities to legislate against no deal or request a revocation of article 50, and frequently failed to take them, most recently three weeks ago. Yvette Cooper’s April bill forcing the government to request a Brexit extension was the notable exception. But we also know there is a parliamentary majority against no deal, because over 60% of MPs opposed it in the indicative votes.

There are currently 312 Conservative MPs in parliament, down five since the 2017 general election. With the DUP, that provides a working majority of just three. This could reduce to two after next month’s hotly contested byelection in Brecon and Radnorshire. Johnson may have the self-confidence of a young Tony Blair, but emphatically not the parliamentary numbers.

There are three available routes to derail no deal. The safest option is to prepare fresh legislation mandating an extension request, with provision for the government to revoke article 50 if the EU declines such a request. Ten Tory MPs supported the motion to seize government time last month, while eight Labour MPs opposed it. It failed by 11 votes. Numerous Tories failed to back it because they did not want to embarrass the leadership contenders and did not consider we had reached a crisis point. After the summer, that will change. As we approach the deadline, the front pages will assemble daily announcements of investment withdrawals and job losses. Public anxiety will grow and the pound will fall. MPs will act, just as they did in April.

The second option is a vote of no confidence. Labour leavers would back it, just as they did in January, and such Conservatives as Dominic Grieve and Ken Clarke have already committed to supporting one as a last resort. Even Philip Hammond has hinted that he might vote against a new Conservative government in order to block a no-deal Brexit. The question is not why Tory colleagues would join them, but why they wouldn’t. Moderates and former remainers know that no deal will bring their constituents misery. They know it will shatter their party. They know they will not be able to hold their seats in the election which follows. What could be in it for them to toe the line? Nobody will thank them for it. A passive acceptance of no deal would eclipse anything else they might have achieved and haunt them to the end of their political lives.

Even by the measure of naked self-interest, MPs will calculate correctly. Johnson’s foolish insistence on a total loyalty to the 31 October deadline necessarily displaces current moderate ministers to the backbenches. Loyalists such as Amber Rudd, Greg Clark and Rory Stewart will have nothing to lose. Minister Margot James has said that she will “do whatever it takes in the end” to stop no deal.

The final option is an emergency revocation. If the EU will not extend or MPs feel too squeamish about bringing down the government, this may be the only way out. The European court of justice has ruled that the UK can revoke in accordance with its “constitutional requirements and following a democratic process”. If the people’s representatives vote for it, it will be politically impossible for the executive – and the EU – to ignore it.

Polls consistently demonstrate that no deal has the support of around a third of voters. In this, parliament directly mirrors public opinion. Not only is there no mandate for it, but there’s no possibility of a happy ending afterwards. The Tory party may now be almost fully radicalised, but the country is not. MPs can choose just one to save – and in the end, will make the right choice.

• Jonathan Lis is deputy director of the thinktank British Influence