Northern Ireland’s largest political party has confirmed Westminster veteran Sir Jeffrey Donaldson as its next leader, setting the scene for a stand-off with the UK government over post-Brexit trading terms that threaten the collapse of the region’s devolved institutions.

Donaldson, 58, was the only candidate to enter the Democratic Unionist party’s leadership race by the deadline of noon on Tuesday. The contest was triggered by last week’s chaotic removal of Edwin Poots, who was appointed just 21 days earlier, following the ousting of Arlene Foster.

“The task ahead is great,” Donaldson said, vowing to “lead unionism into the second century”, a reference to the formation of the region 100 years ago, and singling out the Northern Ireland protocol, which governs the region’s post-Brexit trade with the rest of the UK, as his most pressing issue.

“To that end, I will be speaking with the prime minister at the earliest opportunity to emphasise that it is not realistic to expect stability when every unionist representative in the devolved institutions opposes the Northern Ireland protocol,” the 58-year old added.

The EU and UK agreed to a customs border in the Irish Sea to prevent goods from Britain flowing freely into Northern Ireland, which remains in the EU’s single market. Devised to avoid a border between Northern Ireland and the Republic, the arrangement infuriated unionists who see it as an affront on their identity as UK citizens.

Donaldson warned on Monday that Stormont’s power-sharing government could be at risk if the UK does not take “decisive action” on the protocol, a stronger tone than that struck by Poots, who contemplated tackling the protocol in four years’ time when the regional assembly at Stormont can vote on parts of it.

A UK government official said Whitehall “wouldn’t read too much into” Donaldson’s comments. “An early election [triggered by Stormont’s collapse] is not in anybody’s interests,” the source said, adding that Donaldson was seen as a “more pragmatic figure” than Poots, whose leadership imploded after big promises and catastrophic misjudgments.

The source also downplayed claims from Poots on Tuesday morning that he had secured promises of “significant changes” to the Northern Ireland protocol, in talks with the UK government last week that averted the collapse of Stormont.

Poots was promised nothing beyond what the UK had already publicly committed to, the official said. The UK has said it is exploring all options to ease problems with the protocol, which could restrict Northern Ireland’s access to vital medicines and other products such as chilled processed meat once grace periods expire.

Poots was forced out after he defied an overwhelming majority of the DUP’s Stormont and Westminster representatives and agreed to continue with the regional assembly on terms that included the UK directly intervening to fast-track pre-agreed legislation protecting the Irish language.

“The last number of weeks has been difficult for the party and mistakes have been made,” DUP chair Lord Maurice Morrow said on Tuesday. “Now is the time to move forward in a spirit of humility and mindful that our focus must be on serving the people whom we represent.”