Electoral reform is needed urgently in the UK for the “progressive majority” to have their voices heard, a co-leader of the Green party has said as he announced plans to step down this month. Jonathan Bartley, who has served for five years as co-leader, said he wanted to leave enough time for his successor to take the reins before a possible snap general election before 2024. Bartley has not ruled out standing as an MP but will concentrate on building a progressive alliance across left-leaning parties, which collectively attracted more votes in the 2019 general election than the Conservatives and smaller rightwing and Eurosceptic parties combined, but which fell to a crushing defeat as Boris Johnson took an 80-seat majority in parliament. Under a fair voting system the Green party would have 50 MPs on its current poll ratings, Bartley said, instead of a single MP, Caroline Lucas in Brighton Pavilion. “Once you reform our democracy it opens up huge possibilities,” he told the Guardian after announcing his departure on social media. “There is a progressive majority in this country but we will have Conservative governments as far as we can see because of first past the post.” He accused the government of undermining democracy through a series of actions including changes to voter ID that Labour has said could disfranchise millions of people. “We need to change the democratic system. This government recognises the threat to it from that, which is why they have these moves to change the voting system,” Bartley said. “The government is playing fast and loose with democracy, using our [unwritten] constitution to their advantage in a way that is shameful, ploughing on against public opinion.” The Greens had a record showing in the delayed local elections in May, with a net gain of 91 council seats in the local elections, taking its national total to a record 444. It has a role in running 18 councils, and took council seats almost equally from the Conservatives and Labour: 45 of its seats were won from Tory incumbents and 49 from Labour, with a further four from the Liberal Democrats. Bartley, a south London councillor who is the longest-serving Greens leader, will stand aside at the end of July, and the other co-leader, Siân Berry, will remain in place until the race to replace them concludes. Bartley said he was “hugely proud” of what the Greens had achieved in the five years of his leadership, claiming they had become a “major electoral force, vying to be the country’s third party”. He said he was quitting because of the growing chance that Johnson could call a general election before May 2024, meaning a new leader would be needed quickly to get used to the role and begin planning for the election campaign. Under the Fixed-term Parliaments Act, which Johnson has vowed to repeal, the next general election is due to take place on 2 May 2024. However, MPs can trigger an earlier contest if two-thirds of them vote to do so. Speculation has grown in Westminster that Johnson may decide to go to the country in spring or autumn 2023, wanting an election before many of the tougher economic decisions have to be taken in the aftermath of Covid and before the public inquiry into the government’s handling of the pandemic is likely to have reported. Bartley said leadership was about “empowering and encouraging others” and it felt like the right time to make way for new leaders. He paid tribute to the co-leaders with whom he had served – Lucas for two years and Berry, a London assembly member, for three. Berry paid tribute to her “tremendous colleague” who she said had been “hard-working, thoughtful, kind, collegiate and full of insight and ideas”. She added: “Every member should applaud the huge part he has played in the Green party’s progress over the past five years. His departure leaves many impressive pairs of shoes to fill.” A Green party spokesperson said details and a timetable for the leadership election would be announced in due course.