Nearly 20 years ago, the Good Friday agreement was signed by the UK government, the Irish government and eight of the political parties in Northern Ireland. After decades of violence and bloodshed, during which more than 3,600 people were killed, it provided the basis for the relative peace and development we have seen in Northern Ireland since. The invisible and open border between Northern Ireland and the Republic of Ireland is, as the Irish government has said, arguably “the most tangible symbol of the peace process”.

Against that background, the UK’s exit from the EU undoubtedly presents a significant and unique challenge. That challenge has been on the prime minister’s desk for 20 months, but we are still waiting to hear how she intends to overcome it. The dilemma facing Theresa May is one of her own making. The red lines she laid out in her Lancaster House speech last January – including no customs union and no membership of the single market – are incompatible with the commitments she made later in the year to protect north-south cooperation and to avoid a hard border in Ireland.

Labour wants a deal that protects the economy and works for every corner of the country, including Northern Ireland

The agreement she signed up to in December was a promise to the people of Northern Ireland that there would be “no physical infrastructure or related checks and controls” after Brexit. A solemn promise, rightly made. Yet there are some in her party who, in recent weeks, have argued that the government should abandon the Good Friday agreement and accept the return of a hard border as the inevitable consequence of the Brexit process. The prime minister should have no truck with such reckless talk. The pursuit of an extreme Brexit cannot come at the cost of peace in Northern Ireland.

May is running out of time and running out of road. The time for leadership has come. At the start of the week, Labour showed that leadership by answering the question that this government has ducked for too long. In setting out Labour’s vision for Britain after Brexit, Jeremy Corbyn made clear that Labour would seek to negotiate a new comprehensive UK-EU customs union to ensure that there are no tariffs with Europe and to help to avoid a hard border. Any such deal would also have to ensure the UK would have a say in future trade deals negotiated with the EU. He went on to spell out that Labour would negotiate a new and strong relationship with the single market, with no new impediments to trade and no reduction in rights, standards and protections. This combination is intended to hardwire the benefits of the single market and the customs union into the final agreement and to secure a deal that maintains Britain’s full access to European markets.

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Labour’s priorities are clear: jobs and the economy must come first; not party interests or ideological fantasies. And there is widespread support for Labour’s approach – at home and abroad – including from the Confederation of British Industry, the Institute of Directors and the trade union movement. Labour’s approach is not about what is politically right, it is about what is right for the country. We want a deal that protects the economy and works for every corner of the country, including Northern Ireland.

The test for May when she delivers her speech on Friday afternoon is whether she can follow Labour’s lead. Can she overcome the deep divisions within her government and set out a coherent, credible vision for Britain’s future relationship with the EU after Brexit? Can she stand up to the extreme Brexiteers within her party and lead the country through the most difficult negotiations in our recent history? It’s a test of leadership, strength and resilience. If she fails this test, Labour is waiting in the wings.

• Keir Starmer is Labour’s shadow secretary of state for exiting the European Union