A formal battle for the UK Labour Party leadership edged closer after both main contenders to unseat prime minister Keir Starmer made big moves on Thursday.

Wes Streeting resigned from his role as health secretary, saying he had “lost confidence” in Starmer’s leadership and could no longer serve in his cabinet. Streeting did not declare that he would formally challenge the UK prime minister, but his allies were openly drumming up support among Labour MPs in Westminster.

In his resignation letter, Streeting also said “the debate that comes next ... needs the best possible field of candidates”. This appeared to be a coded way of saying any future leadership contest should include other contenders such as Greater Manchester mayor Andy Burnham, who is currently blocked from running as he is not an MP.

Meanwhile, Burnham unveiled his strategy to solve that very issue by announcing his intention to seek permission from the Labour Party’s ruling national executive committee (NEC) to run in a byelection in Makerfield near Wigan, 30km from Manchester.

Burnham had been seeking a Labour MP to stand aside to let him run ever since the party’s NEC, stacked with Starmer’s allies, blocked his last byelection bid in Gorton & Denton.