**Andy Burnham leads the market at 76.7% because he is the Labour candidate in the Makerfield by-election scheduled for 18 June 2026, a contest widely viewed as the pathway for him to re-enter Parliament and mount a leadership challenge against Prime Minister Keir Starmer.** Labour’s weaker-than-expected local election results earlier in 2026 triggered internal party pressure on Starmer’s position, elevating Burnham—currently Greater Manchester mayor—as the most credible immediate alternative within the governing party. Traders price “No Next PM in 2026” at 13.0% to reflect the possibility that Starmer retains the role through the year or that any transition occurs later. Lower-priced names such as Angela Rayner, Wes Streeting and others trail because none currently holds a comparable near-term route to a parliamentary seat and leadership contest. The outcome hinges on Burnham securing the seat and subsequent Labour parliamentary arithmetic, with the by-election serving as the dominant near-term catalyst.