Leo Varadkar, Ireland’s deputy prime minister, has accused the British government of risking the break-up of the United Kingdom and making “shocking” blunders over Northern Ireland.

Varadkar said Boris Johnson’s administration had been undemocratic and disrespectful and tacitly accused it of being dishonest and dishonourable.

The tánaiste made the sharp attack in a BBC interview on Thursday night, days after the Northern Ireland protocol bill – which could override the Brexit deal – cleared its first hurdle in the House of Commons.

“I think that’s a strategic mistake for people who want to maintain the union because if you continue to impose things on Northern Ireland that a clear majority of people don’t want, that means more people will turn away from the union. It’s a peculiar policy coming from a government that purports to want to defend the union,” he said.

Varadkar, who is due to succeed Micheál Martin as taoiseach later this year, said he found it “shocking and hard to accept” that Downing Street sought unilaterally to change the protocol. “What the British government is doing now is very undemocratic and very disrespectful to people in Northern Ireland because it’s taking that power away from the assembly.”

An honourable government would honour a treaty it had agreed and abide by international law, he said. “It is not normal for a democratic government in a respected country to sign a treaty and then try to pass domestic legislation to override it,” he said.

Varadkar rubbished statements from Liz Truss, the UK foreign secretary, who said the EU’s proposed solutions would worsen bureaucratic impediments. “Well, there are some people clearly who are able to say a square is a circle. That’s just not the facts.”

Asked about a statement from Brandon Lewis, the Northern Ireland secretary, that relations with Dublin were “great”, Varadkar replied: “In my political lifetime, I’ve never seen relations this bad.” London did not want to work with Dublin, was picking fights with Brussels, and not being even-handed in Belfast, he said.

Separately, in a letter to the Financial Times, Adrian O’Neill, Ireland’s ambassador to the UK, rebutted an opinion piece by Truss that had defended the protocol legislation. It would destabilise Northern Ireland by creating a legal and political vacuum, he said.

Meanwhile, Michelle O’Neill, Sinn Féin’s deputy leader and putative first minister of Northern Ireland, on Friday laid a wreath at a cenotaph in Belfast to commemorate British army soldiers – many from Ireland – who died at the battle of the Somme in 1916, an important anniversary for unionists.

Sinn Féin leaders have previously attended first world war commemorations but not in Belfast. O’Neill attended a low-key event before the city’s official commemoration. “As political leaders we have a responsibility to reach beyond our comfort zones and reach out the hand of friendship and to do whatever we can in terms of leadership and healing the wounds of the past,” she said.