Who will be UK prime minister on June 30, 2027?

Burnham took power in the most emphatic way available: a by-election win with a 9,200 majority, then nominations from 379 of the 403 Labour MPs and no opponent. That gives Labour MPs a strong face-saving reason not to turn on him: replacing a leader they chose almost unanimously in July would admit a second mistake after Starmer. His main internal rival, Wes Streeting, ruled himself out and took the defence brief, which binds him to the government.

The pressure point is the bond market, and the government's behaviour shows it knows that. Chancellor John Healey met economists from the primary dealers before the Budget and, by the account quoted in press reports, stressed fiscal credibility and the fiscal rules. That is the opposite of the Truss approach in 2022, when the government announced unfunded tax cuts without forecasts. A leader who is visibly courting lenders is less likely to trigger a self-inflicted crisis, but he cannot control a global sell-off that is lifting borrowing costs everywhere.

Reference class: prime ministers who took office between general elections since 1955, from Anthony Eden through Rishi Sunak. Of 10, 9 were still in office about 11 months later. The only one who was not, Liz Truss, stepped down on her fiftieth day after a market crisis triggered by her first fiscal statement. That puts the base rate at 0.90, and the Truss case is exactly the channel now in view.

The closest analogue is Gordon Brown in 2007, who also took over mid-term, unopposed, with a record share of MP nominations, and survived to the next election despite a rough first year. The analogy breaks on the polls and the markets. Brown began with a large Labour lead; Burnham's Labour is on 24 percent in YouGov’s latest poll, one point ahead of the Conservatives and three ahead of Reform. And Brown's first year came before the financial crisis, while Burnham's first Budget lands with 10-year gilt yields near their highest since 2007.

Start from the 0.90 base rate. Version 1 added 4 points for Burnham's unmatched internal mandate and the absence of any scheduled election, and took off 1 point for Budget and local election risk, landing at 0.93. We correct one claim from that version: it said he had nominations from 94 percent of Labour MPs, but the page it cited says only 'over 80%'. The figure itself holds: AP reported 379 of the 403 Labour MPs, which is 94 percent, our calculation, and we now cite AP.

Version 1 underweighted the market channel. Since then, thirty-year gilt yields first broke 6 percent on October 1 and then reached 6.036 percent on October 7, their highest since January 1998, and the 10-year rose to 5.48 percent, with the Budget set for October 28. We now take off 3 points for the Budget and market risk, not 1, and 1 more point because Labour's lead fell from five points to one in a week of YouGov polling and Kalshi's price for a departure announcement before July 1, 2027 rose from 6 cents to 11. That gives 0.90 plus 0.04 minus 0.03 minus 0.01, or 0.90, our calculation. We do not cut further because the yield rise is global, with U.S. Treasury yields also rising on the day, and a Truss-style exit needs a self-inflicted fiscal shock, not just high rates. Of the remaining 0.10, about 0.07 is an unplanned Labour succession and 0.03 a snap election that produces a different prime minister; a snap election is unlikely while Labour polls 24 percent, and it would have to be lost as well as called.