Greater Manchester Mayor Andy Burnham confirmed he would seek to enter a Labour leadership contest if he wins the June 18 Makerfield by-election, speaking on June 4 on a BBC Question Time special alongside the other candidates for the vote. Burnham also suggested that former Health Secretary Wes Streeting was already effectively running a leadership campaign, while hinting he would consider a potential income tax cut were he to become prime minister.

Burnham's remarks mark his first explicit commitment to challenging Starmer in a leadership contest. The Makerfield by-election was triggered when then-Labour lawmaker Josh Simons stepped down, citing a desire to allow Burnham to return to Parliament in order to participate in an eventual leadership bid. Streeting had already signaled in May that he would enter a contest. Starmer has rebuffed calls from Labour lawmakers to resign after the party's heavy losses in the May 7 devolved and local elections. Labour rules require a challenger to be a sitting lawmaker backed by 20% of lawmakers — currently 81 of 405 — to trigger a contest.