Anti-Brexit campaigners Borders Against Brexit protest outside Irish Government buildings in Dublin Reuters

LONDON — The Irish government has dealt a blow to a key part Theresa May's Brexit plan, describing her proposals for the border between the Republic of Ireland and the United Kingdom as unworkable.

The Irish government is calling for the Irish Sea to become the border between the Republic of Ireland and the UK after Brexit as it does not believe the UK government's current plans will work, the Times newspaper reports.

The Irish Taoiseach Leo Varadkar has warned Prime Minister May that her plans for the border between the Republic of Ireland and Northern Ireland could even harm the peace process.

Simon Coveney, the Irish foreign minister told a meeting of EU foreign affairs ministers: "What we do not want to pretend is that we can solve the problems of the border on the island of Ireland through technical solutions like cameras and pre-registration and so on. That is not going to work.

"Any barrier or border on the island of Ireland in my view risks undermining a very hard-won peace process and all of the parties in Northern Ireland, whether they are unionist or nationalist, recognise that we want to keep the free movement of people and goods and services and livelihoods."

May's Conservative government had been working on a plan that would involve surveillance cameras in order to allow free trade between across Ireland. However, the Irish government wants customs checks to happen at ports and airports instead, creating a new border in the Irish Sea as a result.

If as expected the UK leaves the Customs Union as part of its divorce from the European Union Brexit, a hard border would likely have to be introduced between the Republic of Ireland and the UK. This could harm the peace process as it partly relies on there being no difficulty in goods or people moving between Northern Ireland and the Republic.

Neither the Irish or British government want a return to a physical hard Irish border that would mean checkpoints being operated and customs checks that would affect communities and businesses both sides of the border.

The European Union's chief Brexit negotiator Michel Barnier has previously warned that "frictionless trade" is "not possible" following Brexit.

British government officials have apparently been surprised by the Irish government's change in tone, announced at an EU summit in Brussels last week.

A Whitehall source told the Times: "There is a new taoiseach and a new foreign minister and they’re stamping their authority. We’re being as positive as we can but it’s true to say that their attitude has hardened."

Sir Jeffrey Donaldson, a senior DUP MP told BBC Radio 4's Today programme that his party "would be strongly opposed to the idea that you would create a border in the Irish sea between the island of Ireland and Great Britain."

This is important as the DUP are currently partners with May's Conservative government in a confidence and supply deal in Westminster. Any threat to this informal coalition could bring down the government.

When he was asked whether he could rule out his party's support for the idea, Donaldson said: "Yes, I certainly can. There is no way that the DUP would go for an option that creates a border between one part of the United Kingdom and the other."

Brexit secretary David Davis told MPs last year that he "did not see [a sea border] would be the solution."

The Irish Times reported that the Home Office has not consulted any external officials on the impact of Brexit on the Irish border and Irish citizens living in the UK or the Republic.