Theresa May has just faced yet another torrid period of Tory revolts over her Brexit strategy. David Davis was on the verge of resignation. Government MPs were publicly rejecting her customs backstop fudge. And Boris Johnson became so exasperated that he was telling his supporters that he wanted Donald Trump to take over the negotiations.

It is now clearer than ever that May will fail to deliver the Brexit deal that Britain needs. She cannot command the confidence of her cabinet, of her party, or of the country. Instead of negotiating for Britain, the prime minister is lurching from crisis to crisis, increasing the risk that the talks break down, and we crash out without an agreement. We cannot go on like this.

There are just four months left before the UK needs to strike a deal on our future relationship with the EU. It will be the most important agreement this country has struck in a generation. People’s jobs, their mortgages and their children’s futures are all at stake. So, with the government in such turmoil, parliament must be prepared to step in and take a more central role.

This week MPs will vote on a series of amendments to the EU withdrawal bill. These changes could help steer the government away from an extreme and damaging Brexit. They could protect fundamental human rights, guarantee no infrastructure on the Irish border, and break the deadlock within the government over our future customs arrangements with the EU.

The most important of these amendments is to give parliament a truly meaningful vote on the final deal

The most important of these amendments is to give parliament a truly meaningful vote on the final deal. I coined the phrase “a meaningful vote” – and I have been clear ever since that it must be just that: meaningful. I have been equally clear that if the deal the prime minister strikes with the EU does not meet our six tests, then we will vote the deal down. The amendment upon which MPs will vote tomorrow would deliver on the commitment for a meaningful vote. It would provide a safety valve in the Brexit process. It makes clear that – should the prime minister’s proposed article 50 deal be defeated later this year – it would then be for parliament to say what happens next, not the cabinet. It would, in effect, take no deal off the table once and for all.

Last Friday the government revealed its plan to water down the Lords’ amendment: to turn the meaningful vote into a meaningless vote. Not good enough. We must stand up for the principle of parliamentary democracy and not allow the government’s failure in the Brexit process to be a licence for the UK to crash out of the EU without an agreement. So, even at the eleventh hour, I would urge Conservative MPs to reject the government’s proposition and accept the Lords’ amendment.

This is the most important week of the Brexit process since the triggering of article 50. After months of dithering, delay and government splits, there is a chance for parliament decisively to shape the course of the negotiations. A chance for parliament to have its voice heard. We must get it right.

• Keir Starmer is a Labour MP and shadow secretary of state for exiting the EU