BELFAST, Sept 9 (Reuters) - Northern Ireland"s largest pro-British party threatened on Thursday to withdraw from the region"s power-sharing government within weeks unless big changes were made to the terms of Britain"s divorce deal with the European Union. Democratic Unionist Party leader Jeffrey Donaldson demanded the removal of all new trade barriers between the British province and the rest of the United Kingdom, which Brussels says is impossible. "If the choice is ultimately between remaining in office or implementing the protocol in its present form, then the only option for any unionist minister would be to cease to hold such office," Donaldson said in a speech in Belfast. The power-sharing executive"s fate would be clear within weeks, he said, describing the issue as the region"s worst constitutional crisis in its 100 year history. Northern Irish unionists have been infuriated by the region"s post-Brexit status, which was the most contentious issue during London"s negotiations to leave the EU. To avoid border checks between the British-ruled region and the rest of Ireland, Prime Minister Boris Johnson agreed to let some EU rules remain in force in Northern Ireland, with checks on some goods arriving from the rest of the United Kingdom. London now says the arrangement known as the Northern Irish protocol isn"t working and must be changed; the EU has ruled out any renegotiation, though it says it will look at ways of easing implementation. Johnson"s spokesperson said: "We believe that the challenges the DUP and others have set out illustrates that the protocol in its current form is simply not sustainable". The power-sharing government formed from the main parties supporting and opposing British rule of Northern Ireland is one of the central stipulations of the 1998 peace agreement that ended three decades of conflict in the region. Donaldson said the DUP would also immediately halt cooperation with the government of the Republic of Ireland, another measure called for under the peace deal. It would block any additional steps taken at ports to police the Brexit trade restrictions. "What won"t work is mere tinkering or the introduction of a few flexibilities," Donalsdon said. "A piecemeal solution is not going to work." Donaldson was due later on Thursday to meet European Commission Vice-President Maros Sefcovic, visiting Belfast for talks on how to ease the protocol"s impact. Donaldson, who took over as DUP leader in June, is facing the party"s first defeat in regional elections in 16 years next May, with one opinion poll putting it in third place among unionist parties on 13%. If it is not the largest party, it would no longer have the power to block power-sharing. "The DUP is clearly in panic mode, driven by poor opinion polls," said Mary Lou McDonald, head Sinn Fein, the main party opposing British rule. She described the speech as a "reckless, irresponsible and short-sighted election stunt."