The government is crashing toward the critical stages of Brexit negotiations. In 11 months we leave the EU and enter a transition period. The prime minister can no longer rely on warm platitudes or catchy phrases, but will soon be judged on the reality of the deal she achieves (or worse, the no-deal she ends up with). The success of Brexit now hinges fundamentally on two questions: whether our new relationship with the EU will put the British economy first, and whether the Good Friday agreement, which has brought lasting peace in Northern Ireland, will be respected.

To achieve both outcomes, the UK must negotiate a new customs union with the EU. This would ensure that no Northern Irish border is necessary. It would also maintain the frictionless movement of goods into and out of the country, keeping many of the benefits of trade with the EU.

Customs union Q&A: what is it and can the UK manage without it? Read more

It was a former prime minister who once noted how “it is no small thing to have completed and preserved a customs union covering a market of nearly 300 million people”. No, not the words of Tony Blair, nor Harold Wilson, but of Margaret Thatcher. Oddly, this puts the Iron Lady on the same side as Jeremy Corbyn when it comes to this crucial issue of Britain’s future relationship with the EU (though not much else), and on a different side of the argument to Theresa May. By explicitly ruling out such an aspiration in her negotiating stance, May takes a rather lonely perch against this consensus, confined by the ideological whims of the Tory hard-right.

This is an unenviable position for the prime minister, who looks increasingly exposed to the judgment of history. Politics is, after all, about building coalitions. This is a point not overlooked by MPs from across the political spectrum, who are coalescing around a motion that calls for the government to accede to Labour’s demand for a customs union. The benefits of Labour’s position are so many that most MPs now find it impossible to disagree that it is the only route forward. The motion has found signatories from almost all parties, and was tabled by the chairs of every select committee in the Commons. It leaves those who support British isolation after Brexit looking rather isolated themselves.

Should this cross-party coalition win the vote today it would be yet another symbol of a widening split between parliament and the government over May’s Brexit approach. We have seen the government defeated three times in the Lords, including on the question of a customs union. With legislation still going through the Commons on our future relationship, it seems the voice of parliament will be respected. The case is now overwhelming. The government must back Labour’s position of negotiating a new customs union with the EU, as outlined in full by Corbyn at the beginning of this year, and by Thatcher 40 years earlier.

• Peter Dowd is shadow chief secretary to the Treasury