The British government has become an “advocate for unionism”, the Irish prime minister said yesterday, adding that he had “some difficulties” with the deal agreed to restore power-sharing to Northern Ireland.

Leo Varadkar said that the compromise agreed with the Democratic Unionist Party had led the government to drop its “rigorous impartiality” between nationalism and unionism.

However, he said that the agreement did not cross any of his red lines and it was “really good news” the assembly and the executive were due to be up and running today.

Varadkar was responding to language in the deal which said that the government was “committed to strengthening Northern Ireland’s place in the Union” and had been clear that “we will never be neutral on this issue”.

In Dublin yesterday Varadkar said: “I would have some difficulties with some aspects of it. I don’t like the negative language about the all-Ireland economy and I think it very much puts the British government in the place of being advocates of the Union, whereas in the past they’d signed up to rigorous impartiality.” He added: “None of those things crossed any red lines in my view.

If this is the price, if this is what has to be accepted in order to allow powersharing to resume, I think that’s worth it.”

Sir Jeffrey Donaldson, the DUP leader, is due to be sworn in as deputy first minister today and Sinn Fein’s Michelle O’Neill will become the first nationalist first minister.

Donaldson was challenged by a former attorney-general for Northern Ireland who rejected his claim that the deal in effect removed the Irish Sea border.

John Larkin KC was commissioned by opponents of the agreement to assess the legal effect of the measures and said that they would not remove customs and regulatory barriers and did not ensure “zero checks and zero paperwork” for GB goods destined for Northern Ireland.

The opinion was commissioned by a group including Jim Allister, the Traditional Unionist Voice leader, and Baroness Hoey, a former Labour minister.

Allister said that the existing post- Brexit arrangements would attempt to deliver a united Ireland by stealth.