Theresa May has got her transition agreement. To get there she has had to surrender on just about every one of the issues she and her ministers once told us were key sticking points.

The UK will be paying billions over to the EU despite having no say in its decisions, free movement will continue, the European court of justice will be able to issue instructions to British courts for at least another decade, the common fisheries policy will still apply to Britain – the list goes on.

Don’t let the Brexiters turn Ireland into a new Cyprus | Andrew Adonis Read more

The one thing that the transition agreement has come nowhere near resolving is the biggest issue of all: the network of future relationships across these islands.

By keeping the UK in the customs union and single market for another 20 months, the transition agreement puts back the need to answer the big questions about the relationship between the UK and the Republic of Ireland, but it does not remove it.

The damage a disorderly and ill-thought-out Brexit could do in Ireland is enormous. We are often told Brexit threatens to “reimpose” a so-called hard border on the island of Ireland, but that understates the problem. Because the economic border that a hard Brexit would impose on Ireland would be the hardest ever.

Play Video 1:00 Michel Barnier: Irish border checks 'unavoidable' under Brexit – video

Not only would Northern Ireland and the Republic have different currencies but different regulatory regimes and customs barriers as well.

But this is Ireland, and while the economy is one thing, the symbolism is another. The openness of the Irish border is a hugely important sign of the continuing successes – despite all the problems – of the peace process cemented into the Good Friday agreement. More than that, it demonstrates, every day, that different identities, histories and jurisdictions can coexist on the island without threatening each side’s integrity or legitimacy.

I’m pleased my party has taken a big step by backing continued customs union membership, but we need to go further

Given the immense suffering, in Britain as well as in Northern Ireland and the Republic, during the Troubles, we mess with these symbols at our peril. The chief constable of the Police Service of Northern Ireland says a hard border would put his officers’ lives at risk and it seems like madness that we would even contemplate doing that.

More than that, when the British and Irish governments signed the Good Friday agreement we were entering into a contract with the people of Ireland, north and south. It is disgraceful that we think we can break that without their consent.

If we insist on leaving the EU then there is realistically only one way to honour our obligations under the Good Friday agreement and that is to remain members of both the customs union and the single market. I’m pleased my party has taken a big step in this direction by backing continued customs union membership, but we need to go further.

Labour needs to do more than just back a soft Brexit or guarantee a soft border in Ireland. Given that it is increasingly obvious that the promises the Brexiters made to the voters – especially, but not only, their pledge of an additional £350m a week for the NHS – are never going to be honoured, we have the right to keep asking if Brexit remains the right choice for the country. And to ask, too, that the country has a vote on whether to accept the terms, and true costs of that choice, once they are clear. That is how Labour can properly serve our democracy and the interests of our people.

• Owen Smith is shadow Northern Ireland secretary