Nobody knows the tragedy of a small island divided against itself better than a Cypriot. My father came to Britain in search of a better life. My aunts, uncles and cousins fled here in search of safety as Cyprus’s Greek and Turkish populations fell into open hostility. Decades later, the scars of that conflict run deep and raw still. My father’s homeland is cut in two, with families still separated from their homes and land by a border that makes no sense but that stubbornly clings on. Nicosia is the last European capital to be split in two by a national boundary. The enrichments of trade and friendship alike have been squandered, to the misery of many and the benefit of few.

As I visit Ireland this week, my ancestral home is in my thoughts. Alongside Nick Clegg and Michael Heseltine I am meeting the Irish deputy prime minister, the Brexit envoy and representatives of businesses that work across the Republic of Ireland and Northern Ireland – to try to understand the Irish position, to think about how we can stop a hard border and to reassure our nearest neighbours that not all British politicians view them as an inconvenience or a dominion.

For me, no one aspect of the Brexit debate displays so markedly the monomania of many Brexiteers as does the Irish question. Peace in Northern Ireland, so delicately achieved by governments of both political stripes, led to prosperity and positivity. Central to the success of the peace process was an understanding that Ireland was not a colony to be subdued but a partner to be cherished, and that Ulster is equally treasured by London and by Dublin, while fully part of the United Kingdom. An open border – across which could flow people and products alike – was key.

But all of that careful balance is now being jeopardised to slavish ideology. Our foreign secretary, Boris Johnson, privately concedes that while he aspired to a “soft border” – and promised one – there will in fact have to be some form of checks and customs. Jacob Rees-Mogg and his fellow Faragists contemptuously instruct Ireland to break both with its partners across Europe and with international law by either ignoring the need for a border, if the UK is outside of the single market, or by following us out. Meanwhile, fudge after fudge is used to distract attention from this issue in our negotiations, to leave it till later and hope that something will turn up.

The dismissive language of ‘scaremongering' is used to shut down concerns about the threat a border will pose to peace

In this, as in so much else, I’m afraid reason has proved correct and bombast has been found wanting. During the referendum, we worrying remainers were laughed at for warning that a vote to leave would mean a hard border. Now we know that it does. Now, the same dismissive language of “scaremongering” is used to shut down concerns about the threat that border will pose to peace and to prosperity. I hope that this time we will be proven wrong. But the cavalier attitude of Brexiteers is shocking to anyone who genuinely cares.

The Good Friday agreement brought much hope to Cypriots the world over. Maybe this would serve as the blueprint for our divided island, they thought. The fact that it has not, yet, is a tribute to just how difficult and fiendish such disputes are to solve. Having achieved something so special as peace, it is criminal to throw it away on a whim. My message to the Irish today will be that most of us in Britain are resolved never to let that happen.

• Andrew Adonis is a Labour peer, former transport secretary and former chairman of the National Infrastructure Commission