The future of Northern Ireland’s power-sharing government came under threat on Monday after nationalist party Sinn Féin warned of the danger of “broken promises” and failed to immediately agree to a new administration following the departure of unionist first minister Arlene Foster.

Foster, who was ousted as leader of the Democratic Unionist party a month ago, formally stepped down as head of the region’s devolved administration on Monday, kicking off a seven-day countdown for the DUP and Sinn Féin to agree new leadership.

The Belfast drama comes at a crucial time as the EU and the UK trade blows over how to resolve an escalating impasse over the Northern Ireland Protocol which imposed a customs border between Northern Ireland and Great Britain in the aftermath of Brexit.

Edwin Poots, the DUP’s new leader, warned on a morning radio show that Sinn Féin would be “putting our very peace at risk” if they jeopardised the power-sharing at Stormont at such a critical juncture, words that invoke memories of Northern Ireland’s historic troubles, that saw more than 3,600 people killed, and riots on the streets in April.

The biggest obstacle in early talks about Northern Ireland’s change of leadership are provisions concerning the Irish language in the New Decade, New Approach package underpinning the current government.

The deal creates an Irish language commissioner and promises to “enhance the development and use of the Irish language by public authorities”. Some unionists fear this will lead to street signs in their neighbourhoods in a language they say has been “weaponised” by Sinn Féin. They also worry that Catholics who speak Irish would be at an advantage in recruitment for public sector jobs.

“The previous leadership in the DUP had undertaken to fulfil those commitments [in the NDNA deal] . . . and now the current leadership of the DUP are telling us that they won’t honour that,” Sinn Féin finance minister and chief negotiator Conor Murphy told reporters as he entered the assembly at Stormont for Foster’s noontime departure speech.

His party leader, Northern Ireland’s deputy first minister Michelle O’Neill, made veiled reference to the row in her tribute to Foster, telling the assembly: “You can’t build power-sharing from broken promises. You must deliver upon agreements that are made.”

The mechanics of the assembly means that if either the first minister or deputy first minister step down, both positions are retired and parties must then nominate their candidates. If Sinn Féin refuses to nominate a deputy first minister within that timeframe, a snap assembly election will be called.

Earlier on Monday, Poots told BBC Radio Ulster that “time will tell” whether the legislation on the Irish language would be brought in during the course of this parliament and stressed that it was “not appropriate” for Sinn Féin to set “preconditions” on reinstating Northern Ireland’s political leaders. The DUP will nominate Paul Givan, a longtime ally of Poots, as first minister.

Sinn Féin are leading in the polls but a snap election could still hurt them. “They don’t want to be the ones that collapse the institutions for a second time,” said Professor Feargal Cochrane of the University of Kent, referencing Sinn Féin’s decision to pull out of government over the “cash for ash” renewable energy scandal in 2017.

“The rhetoric has become very high octane,” he said. “It’s ‘will an Irish language act cause people to either die because the [hospital] waiting lists got longer or because Covid hasn’t had a local hand because the executive collapsed?’.”

In a widely praised speech peppered with humour despite her brutal ousting, Foster stressed that the Irish language measures were part of a “cultural package”, which also included protection for veterans and broader investment for “Ulster Scots” broadcasting.

“I encourage all of you to do all of it,” she said to the house. “To take it forward in its totality, to speak of an implemented-as-one complete independent package,” warning of the dangers of “one step forward for some and one step back for others”.