Ireland’s prime minister has warned that Brexit threatens the Good Friday agreement and could “drive a wedge” between Britain and Ireland.

Leo Varadkar also sought to reassure unionists, who fear that his government is using the moment to push for a united Ireland, that he does not have “a hidden agenda”.

The taoiseach was joined on Tuesday by the former Sinn Féin leader Gerry Adams and the British ambassador, Kim Darroch, at an event in Washington to mark the 20th anniversary of the peace deal.

Reflections on the scale of that “historic achievement” fell under the shadow of Britain’s planned exit from the EU.

Brexit had “undoubtedly changed the political weather”, Varadkar told an audience including US politicians at the

“To me, Brexit is a threat to the Good Friday agreement simply because it threatens to drive a wedge between Britain and Ireland, between Northern Ireland and the Republic of Ireland, and potentially between the two communities in Northern Ireland. And that’s why we must do all that we can to make sure that those wedges, that that risk, does not become reality.”

Q&A

Why is Dublin opposed to the idea of a hard border?

Ireland’s taoiseach, Leo Varadkar, has been much more sceptical than the UK about the potential for avoiding border posts via virtual checks on importers. Whilst agreeing with British ministers and EU negotiators that it is inconceivable for there to be a return to a hard border with the north, Dublin argues that the best way for the UK to achieve this would be by permanently remaining in a customs union with the EU and seeking single market membership like Norway through the European Economic Area. The UK has conceded that some of this will be necessary in its interim phase after Brexit, but hopes clever technological solutions can allow it have looser economic links in the long run. Varadkar is not alone in being sceptical about whether such a cake-and-eat-it customs and trade strategy is viable.

Varadkar continued: “I think for unionists – and I take no pleasure in this – it also creates risks for the union itself because it asks Scotland and Northern Ireland to leave the European Union even though the majority of people in both those countries voted not to do so.”

Brexit has raised fears of a return to a hard border between the Republic of Ireland and Northern Ireland reminiscent of the Troubles. Last month it emerged that officials from the UK and the EU are devising a plan to, in effect, keep Northern Ireland in the customs union and single market to avoid this outcome. But that led to calls from Scotland for a similar arrangement.

Meanwhile the Irish government’s efforts to maintain a soft border, in particular its demand that Northern Ireland continues to align with many EU regulations, has spooked unionists. Last year the Irish foreign minister Simon Coveney said he would like to see a united Ireland in his “political lifetime”.

The Democratic Unionist Party (DUP) described the remark as “aggressive” and condemned “the megaphone diplomacy” of Dublin.

On Tuesday, in a message aimed at unionists, Varadkar said: “I know many are concerned, perhaps worried, perhaps even angry at some recent political developments and I want to recognise that recent statements and recent actions by Irish nationalists, including the Irish government, about Brexit have been seen by some as disrespectful and by others as intrusive or interfering.

“And if that was the case, I want to make very clear that that was certainly not our intention, and I want to repeat that we have no hidden agenda. We do not desire a border in the Irish sea any more than we want to see a border between Newry and Dundalk, or between Holyhead and Dublin.”

Britain has insisted it opposed a hard border. Darroch told the audience: “The prime minister said in December last year that the UK remained committed to protecting north-south cooperation and to its guarantee of avoiding a hard border. Any future arrangements must be compatible with these overarching requirements.”

But he was challenged by Colum Eastwood, the leader of the Social Democratic and Labour Party, Sinn Féin’s main nationalist rival, who said: “We are facing an enormous threat, one that we haven’t faced in a long, long time. It’s called Brexit. There are people who are using the Good Friday agreement, who are threatening to rip up the Good Friday agreement, just to make their negotiation with the European Union that little bit easier.”

In recent weeks some pro-Brexit MPs have questioned whether the Good Friday agreement has run its course, pointing out that Northern Ireland has been without a power-sharing government since January last year. Sinn Féin pulled out, arguing that it was not being treated as an equal partner by the DUP. Each side has blamed the other for repeated failures to strike a deal.

But Eastwood said: “We have to understand what has happened. The instability that in our institutions in Northern Ireland is exactly the same instability that’s been created by Brexit. The two things are not separate and I’m glad to hear the British ambassador saying that he doesn’t support a hard border in Ireland.

“But I want to hear from the British government what they’re actually going to do about that because their current position does not stand. It does not add up. It does not work. We cannot achieve frictionless trade and movement on the island of Ireland if we continue down the road the British government wants to continue down.”

Adams also raised the alarm, noting that the majority of people in Northern Ireland opposed Brexit, yet the DUP is propping the minority Conservative government and supporting its Brexit legislation in Westminster. “This threatens the Good Friday agreement,” he said. “It’s a direct assault on the Good Friday agreement.”

In a throwback to fiery rhetoric of old, Adams declared: “I believe the British government has no right to be in Ireland, never had any right to be in Ireland and never will have any right to be in Ireland.”

Adams reiterated the Sinn Féin stance that there cannot be a return to British direct rule and added the best option was for the return of the political institutions. “It might well be that the power-sharing institutions and all-Ireland political architecture may not be restored in the medium term.”

Adams described the Good Friday agreement as a defining moment in Ireland’s recent history but not a settlement. “It never pretended to be. It is an agreement on a journey. Not the destination.”