Theresa May warned against becoming 'too close' to DUP by incoming Irish PM Leo Varadkar

Ireland’s new prime minister has warned Theresa May that doing a deal with the Democratic Unionist Party (DUP) could put the Good Friday Agreement at risk.

Leo Varadkar, the incoming Taoiseach, said the British government should not get “too close” to any Northern Irish party.

“Our role as governments is to act as co-guarantors (to the Good Friday Agreement), not to be too close to any particular party in the North, whether it’s nationalist parties or unionist parties,” he said. “That’s certainly something I will emphasis in any contacts that I have with the Prime Minister.”

His comments came as Ms May prepared to finalise a coalition agreement with the DUP, which would see them support the minority Conservative government.

Mr Varadkar, who won a Fine Gael party leadership contest earlier this month, nonetheless said the DUP could have a positive effect in softening Brexit.

“They want to maintain the common travel area and want to maintain free trade between these islands and Britain and Europe”, he said. “So I do think there’s an opportunity to soften Brexit. But that all remains to be seen.”

Ireland’s outgoing Taoiseach, Enda Kenny, who is due to be replaced by Mr Varadkar this week, has also expressed “concern” over Ms May’s plans to strike a deal with the DUP to help her secure a parliamentary majority.

Over the weekend, he tweeted: “Spoke with Prime Minister May – indicated my concern that nothing should happen to put Good Friday agreement at risk, and [the] absence of nationalist voice in Westminster.”

The power sharing agreement in Northern Ireland collapsed in January after allegations of a financial scandal involving DUP leader Arlene Foster. However, it remains the largest party in the Northern Ireland Assembly.

Under the Good Friday Agreement, the British government must display “rigorous impartiality” in disputes in Northern Ireland and is responsible for mediating between the Northern Irish parties.

Some commentators have suggested that the fact Theresa May is likely to be propped up by one of the main parties means it will be impossible for the Conservative government to fulfil this role.

Alastair Campbell, Tony Blair’s former director of communications, said the Prime Minister was “playing with fire” in attempting to cling to power by making a deal with the DUP.

“The British and Irish governments are the mediators between the Unionists and the Nationalists,” he said. “How can they be the mediator when the Unionists are now being brought into government?”

He had earlier accused Ms May of “playing fast and loose, on Brexit, on Margaret Thatcher’s greatest achievement the single market and now Tony Blair and John Major’s greatest achievement which is the peace in Northern Ireland".

He added: "She is putting that at risk with a sordid, dangerous distasteful deal."

It comes as Ms May prepares to welcome Ms Foster to Downing Street for talks to agree the final details of the deal between the two parties.

The arrangement is likely to be one of “confidence and supply”, in which the DUP will support the Tories on a vote-by-vote basis and agree to back them on key votes such as the Queen’s Speech and the budget.