Sir Jeffrey Donaldson has demanded the removal of the Northern Ireland protocol, in his first speech as leader of the Democratic Unionist party. The MP for Lagan Valley took a hard line over the Irish Sea border on Thursday, calling it the “greatest threat to the economic integrity of the United Kingdom in any of our lifetimes”. Donaldson spoke in Belfast hours after being ratified as the DUP’s third leader in three months, a cycle of party turmoil that continued with the resignation of a Stormont assembly member. Alex Easton, an assembly member for North Down, announced he was quitting the party after 21 years in despair at the infighting. “I have had to stand back and watch as colleagues tear themselves apart, brief against other colleagues and run to the media in order to hurt each other on a daily basis,” Easton said. “There is no respect, discipline or decency, I have just had enough.” The resignation overshadowed an inaugural keynote address in which Donaldson attempted to draw a line under ructions that led to Arlene Foster being ousted as party leader in April, followed by a revolt against her successor, Edwin Poots, last month. “I think that we can all agree that the last few months have not been the proudest in the history of the DUP,” Donaldson said. “When we should have been focused on the needs of wider society, we have been consumed with the internal politics of the party.” The rows had strained the patience of the public and the party’s supporters, he said. “As the new leader of the DUP, I want to apologise to our supporters and to the public for that. More importantly, I want to draw a line under it and to move on.” Donaldson, who plans to swap his Westminster seat for one in Stormont to become Northern Ireland’s first minister, said he would lead opposition to the protocol in the Brexit deal that requires checks on goods from Great Britain. The Irish Sea border threatened not just living standards but the constitutional integrity of the United Kingdom, he said. “In the weeks ahead our goal is to remove the Irish Sea border and to preserve and protect the internal UK market.” The comments appear to represent a hardening from his previous position that the UK government needed to “deal” with the protocol. Police across Northern Ireland are preparing for loyalist marches next week that will commemorate 17th-century battlefield victories and protest against the protocol.