LEO Varadkar has warned the British government that imposing a policy on Northern Ireland that is not supported by the majority will "further reduce support for the union".
The tánaiste said Brexit had "weakened middle-ground support for the union" and that unilateral action from the UK on the protocol will "weaken it even more".
The Fine Gael leader's remarks appeared in an op-ed published by the Guardian yesterday.
In the piece, he recalled his October 2019 'walk in the Wirral' with Boris Johnson, which ultimately led to the British prime minister's "oven ready" deal for withdrawal from the EU, including the protocol.
"Since then, Brexit has taken place, and the protocol is now part of a legally binding international agreement," the tánaiste wrote.
"It is international law. In its final form, it was co-designed by the UK and the EU. It was adopted by the British parliament, the European parliament and the Irish parliament, as well as the governments of 26 other member states. That’s not an easy thing to do."
Mr Varadkar said the protocol is working and that the north is outperforming the rest of the UK economically, while cross-border trade has increased.
"The protocol was even beginning to gain acceptance – albeit without enthusiasm – from some unionist politicians," he wrote.
"It is broadly supported by Northern Ireland’s business community, which has access to the British market, as well as the European Union and its 450 million consumers."
But he says the British government has "changed tack" by threatening to unilaterally suspend parts of the protocol, prompting concern among the region's business community that they'll lose out on investment opportunities.
The Fine Gael leader said a "clear majority" of MLAs and four of Stormont's five main parties favour of retaining the protocol, "either as it stands or in a modified form", while those who want it scrapped are in a minority.
He said the concerns of unionist community around the protocol could not be dismissed.
"Nobody wants checks or controls on trade within their own country, even if it was an inevitable and forewarned consequence of the Brexit that many of them supported," he said.
"The protocol can be improved and modified. The EU has been very flexible already. The European commission has been engaging diligently with the British government to resolve these concerns for years."
The tánaiste said the EU would engage with the UK for as long as the latter was willing.
"But negotiating with a partner that is willing to break agreements and change its mind on what it wants is not easy," he wrote.
"The EU’s flexibility and good faith have not been reciprocated by the UK government. This is breeding mistrust in EU capitals."
Mr Varadkar said the Ireland had been open to other solutions, including a customs union, a closer relationship with the single market, a European free trade area and the backstop, that would have eliminated the need for checks on goods arriving in the north from Britain.
"Perhaps in the future, Britain may reconsider these options, but I know they are not options at the moment," he wrote.
He also highlighted how a pro-union British prime minister and his government "must understand the consequences of imposing a policy on Northern Ireland that is not supported by the majority of the people there".
"They must recognise that this will further reduce support for the union, in my view. Brexit weakened middle-ground support for the union in Northern Ireland. Unilateral action on the protocol will weaken it even more," the tánaiste said.
He said trust could be restored and progress can be made but that talk of scrapping the protocol was counterproductive.
