BELFAST, June 22 (Reuters) - The next leader of Northern Ireland’s biggest political party warned on Tuesday it was “not realistic to expect stability” in the British province while post-Brexit trade rules agreed with the European Union remain in place. Jeffrey Donaldson"s unopposed nomination will make him the third person in a tumultuous few weeks to lead the pro-British Democratic Unionist Party (DUP) during crucial talks about the trade arrangement known as the Northern Ireland Protocol. He has pledged to pressure Prime Minister Boris Johnson"s government to ditch the rules which effectively treat Northern Ireland as part of the EU for customs purposes. "I will be speaking with (Johnson) 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," Donaldson said in a statement. "The government and those who claim to be protectors of peace and stability must step up and deal with the protocol in a manner which respects the constitutional and economic integrity of the United Kingdom." DUP lawmakers ousted then leader and Northern Irish First Minister Arlene Foster in late April in favour of Edwin Poots, who stepped down last week after just three weeks in the role. Donaldson"s candidacy will be formally ratified by party officials next week. The leadership changes have added to political instability in Northern Ireland, which still faces the threat of a snap election over a row between the DUP and Irish nationalist rivals Sinn Fein over the introduction of Irish language rights. The two parties make up a power-sharing executive, put in place by the 1998 peace deal that ended 30 years of sectarian violence in Northern Ireland. Donaldson, a long-standing DUP lawmaker who is their chief in the British parliament, said on Monday that failure to deal decisively with the protocol would have consequences for that arrangement. The protocol was tailor-made to avoid politically sensitive checks on goods from Ireland crossing the United Kingdom"s only land border with the EU. But it is deeply unpopular with supporters of the union with Britain because it has created trade barriers with the rest of the United Kingdom. Poots, who had been seen as more hardline than Donaldson in his opposition to the protocol, said on Tuesday that Britain"s Northern Ireland minister had assured him there would be "very significant" changes to the protocol by early July. Britain"s Brexit minister David Frost said he would not comment on private conversations when asked about Poots" remarks at a British parliamentary committee.