As we approach the 20th anniversary of the Good Friday agreement, it’s sickening to see hard Brexiteers like Boris Johnson undermining it.

That a British foreign secretary would compare the boundary between London boroughs to the border on the island of Ireland speaks volumes. His comments showcase the cavalier approach this government has taken to the Northern Irish peace process.

The threats facing Northern Ireland – of a hard border and the unravelling of the Good Friday agreement – are the product of the government’s own self-defeating red lines, and the reckless demands of the hard Brexiteers. And while they’re quick to attack the position of the European commission, which is published in a draft legal document today, ministers haven’t offered any substantial alternative.

Play Video 0:35 'Like the congestion charge': Boris Johnson on Irish border checks post-Brexit – video

Trade unionists know how precious – and fragile – peace can be. Because over many years, the TUC worked with friends in the Irish Congress of Trade Unions (ICTU) to help achieve it.

Quick guide Why is the Irish border a stumbling block for Brexit? Show Hide Counties and customs Inside the EU, both Ireland and Northern Ireland are part of the single market and customs union so share the same regulations and standards, allowing a soft or invisible border between the two.

Britain’s exit from the EU – taking Northern Ireland with it – risks a return to a hard or policed border. The only way to avoid this post-Brexit is for regulations on both sides to remain more or less the same in key areas including food, animal welfare, medicines and product safety. The 'backstop' in Theresa May's Withdrawal Agreement was intended to address this - stating that if no future trade agreement could be reached between the EU and the UK, then rules and regulations would stay as they are. This has been rejected by Brexit supporters as a 'trap' to keep the UK in the EU's customs union, which would prevent the UK striking its own independent trade deals. There are an estimated 72m road vehicle crossings a year between Northern Ireland and the Republic of Ireland, and about 14% of those crossings are consignments of goods, some of which may cross the border several times before they reach a consumer. Brexit supporters say this can be managed by doing checks on goods away from the border, but critics say it will be difficult to police this without any physical infrastructure like border posts or cameras, which could raise tensions in the divided communities of Ireland.

Interactive: A typical hour in the life of the Irish border Photograph: Design Pics Inc/Design Pics RF

In the 1980s and 90s, unions led massive peace rallies, bringing together working people from all communities who had had enough of violence, inequality and division. And in the 90s, trade unionists were closely involved in the process that eventually led to the peace treaty.

Foremost among them was Unison’s Inez McCormack, the first woman to chair ICTU’s Northern Irish Committee and the first female president of ICTU. McCormack founded and led a coalition of trade unions and other activists, calling for equality, human rights and fair employment practices to be enshrined in the deal.

Throughout the Troubles, trade unions tried to build bridges between working people from different communities. And at the Good Friday agreement referendum, unions came out in support of the deal and encouraged members to vote yes. We knew what an important moment it was – and that unions couldn’t stand on the sidelines. And of course, voters across the island of Ireland felt the same way, backing the agreement with vast majorities.

Since then, trade unionists have done all we can to support the peace treaty. Not only because it’s the best way to promote jobs and a stable economy, but because working people in Northern Ireland are entitled to peace, prosperity and civil rights.

Over the last few weeks we've seen a shamless assault on the Good Friday agreement by extreme Brexiteers

It was in that spirit that, on stage for the televised debate at Wembley in June 2016, I warned that Brexit would present an extraordinary threat to the Good Friday agreement and to Northern Irish communities.

I really wish that I’d been proved wrong. But instead, over the past few weeks in particular, we’ve seen a shameless assault on the agreement by extreme Brexiteers.

Owen Paterson, a former secretary of state for Northern Ireland, supported the view that the agreement has “outlived its use”, while Labour MP Kate Hoey claims it’s “unsustainable”. And let’s not forget that cabinet minister and arch-Brexiteer Michael Gove previously called the agreement a “mortal stain”, comparing its negotiation to the appeasement of the Nazis in the 1930s.

These are unjustifiable attacks on a deal that – although not perfect – ended a devastatingly violent 30-year conflict and gave new hope to hundreds of thousands of people in Northern Ireland. But while these comments are reckless and offensive, I think the indifference of the rest of government – typified by the foreign secretary – disturbs me even more.

Ministers have known all along that their hard Brexit plans are incompatible with the Good Friday agreement. They know that leaving the single market and customs union will destroy jobs and livelihoods in Northern Ireland, and that imposing a hard border will devastate the economy and threaten the peace.

And they are yet to offer any kind of meaningful plan to offset the damage – not helped by their parliamentary dependence on the Brexit-backing DUP. All Theresa May has offered is the flimsy promise that “there will be no return to the borders of the past”. Working people in Northern Ireland deserve better than that.

So the prime minister should explain how we can have regulatory divergence from the EU, but still allow free movement of people and goods on the island of Ireland and protect the agreement. That’s what the legally binding withdrawal agreement with the EU requires of her. The British government needs to make the next move. If there’s a viable alternative to what the European commission are proposing, May needs to set out exactly what it is.

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At the TUC, we’ve identified key tests for the Brexit deal – focusing on jobs, workers’ rights and the Northern Ireland peace process – and measured all the available Brexit options against them. It’s clear that to protect the Good Friday agreement we need to stay in the customs union (and it looks like the best protection will come from staying in the single market, too).

As journalist Fintan O’Toole neatly summarised in the Observer, “You can have hard Brexit or the Belfast [Good Friday] agreement, but you can’t have both.”

It’s clear that the extreme Brexiteers have made their choice. But so have trade unions across the UK and Ireland. Whatever happens, we’ll keep standing up for the Good Friday agreement, for working people in Northern Ireland, and for peace.

• Frances O’Grady is the TUC general secretary