Arlene Foster is facing a revolt from Democratic Unionist party (DUP) members that threatens to topple her as party leader and first minister of Northern Ireland. Multiple DUP assembly members signed a letter on Tuesday calling for an unprecedented leadership contest, according to party sources. A leadership challenge would add further turmoil to a region already rattled by street disturbances linked to the post-Brexit Irish Sea border, which unionists fear will weaken Northern Ireland’s position in the UK. It could also write the political epitaph of Foster, who became the first woman and the youngest person to lead Northern Ireland and the DUP in 2015. Acrimony from Brexit and feuds with Sinn Féin, which collapsed power-sharing for three years, scarred her tenure. DUP members are angry at Foster’s handling of the Northern Ireland protocol, the part of the Brexit deal that imposes checks on goods coming from Great Britain. Foster briefly endorsed the arrangements in January only to row in behind outspoken MPs like Ian Paisley and Sammy Wilson who urged a campaign of resistance. Party hardliners want bolder tactics against the protocol. The party’s Free Presbyterian religious base was also upset at Foster’s decision last week, along with two of her ministers, to abstain on an assembly vote to ban gay conversion therapy. DUP sources told the Guardian the letter, which has not been made public, had been signed by most of the party’s 27 assembly members. The Belfast News Letter, which first reported the letter, said at least 21 assembly members had signed along with four of the party’s eight Westminster MPs. A terse statement from the DUP’s central office did not confirm a leadership challenge but said party officers oversaw the conduct and organisation of its internal democratic electoral processes. “Whilst understanding that there will be from time to time public interest in party processes, these issues, in the first instance, are matters for members of the party and we are not able to make any further comment at this time,” it said. The Press Association reported that Foster cancelled a planned meeting on Tuesday evening with the Northern Ireland secretary of state, Brandon Lewis. The DUP does not usually depose leaders. The late firebrand preacher Ian Paisley founded it in 1971 as a bulwark of British and Ulster Protestant identity during the Troubles. He handed power to Peter Robinson in 2008, who stepped aside for Foster in 2015. The voting system to choose a new leader, which has never been used, puts the decision in the hands of a few dozen people: assembly members, MPs and peers. There is no obvious successor. Jon Tonge, a University of Liverpool politics professor who is an authority on the DUP, flagged Gavin Robinson, the party’s East Belfast MP, as a possible contender. Foster’s authority ebbed in recent months. In Westminster, Paisley, Wilson and other MPs made solo runs that ignored stated party policy. In Stormont some DUP assembly members pushed for more aggressive actions against the protocol to compel action from Boris Johnson and the European commission. Foster – and the rest of her party – have been haunted by the rousing welcome they gave Johnson at their party conference in 2018 when the former foreign secretary promised to fight any attempt to impose a sea border. The DUP smoothed his subsequent path to Downing Street by rejecting Theresa May’s Brexit deal. Foster grew up near the border in rural County Fermanagh where Protestants felt besieged by IRA attacks during the Troubles. Her father, a farmer and part-time police officer, was shot and badly injured outside the family home. Foster, a solicitor by training, enjoyed a brief honeymoon after becoming party leader and first minister in 2015. However bruising battles with Sinn Féin and the so-called cash-for-ash scandal collapsed the power-sharing executive. Critics said she frittered away unprecedented levels of goodwill and made strategic errors over Brexit. “Her intransigence, petulance, arrogance, lack of generosity, and political myopia have been catastrophic for unionism,” tweeted Deirdre Heenan, a social policy professor at Ulster University. • This article was amended on 28 April 2021 because an earlier version, referring to the DUP party conference in 2018, described Johnson as the “then foreign secretary”. At the time, November 2018, Johnson was no longer in that role.