Westminster by-elections

Next up: Clacton on 13 August 2026, where Nigel Farage resigned to re-contest his own seat and the five main opposition parties are refusing to stand. The 18 June 2026 round was a split verdict: Labour held Makerfield (Andy Burnham) against a UKE forecast that wrongly called Reform; the Conservatives gained Aberdeen South in an upset no area signal saw coming; and the SNP held Arbroath and Broughty Ferry exactly as the Holyrood signal predicted.

Where real constituency polls existed (Makerfield only), they beat every model. The two seat models that ignore polls (our published forecast and Electoral Calculus) called Makerfield for Reform, wrongly. On the two Scottish seats no one polled, and Aberdeen South's Conservative gain beat the entire field. Avg share MAE is across the seats each forecaster issued a numeric call on (markets and Ballot Box Scotland gave winners only). Electoral Calculus's figures are its standard general-election seat model, not bespoke by-election forecasts.

Westminster by-elections are rare; council by-elections happen most Thursdays and are the highest-frequency real-vote signal between general elections. Across the 59 ward contests since 19 June 2026, Reform UK made 9 gains, held 5 of its own seats and lost 2: Llangennech to Plaid Cymru, and Old Catton in Norfolk to the Conservatives, where Reform's vote fell to third. The 16 July round was the first real sign of Reform slipping back from its May 2025 county peak in these low-turnout contests, even as it picked up the force-wide Norfolk Police and Crime Commissioner. That mean-reversion is exactly why our Lancashire unitary model swings each ward off the 2025 county result rather than freezing it. The earlier gains came off Labour, the Conservatives and independents across England, Wales and, notably, Scotland (Larkhall, South Lanarkshire, under STV).

Five further contests polled on 23 July 2026 (including two Grays seats in Thurrock that Reform is defending after its own councillors resigned); results due from 24 July.

Winner and previous-holder parties as published by ALDC and the aggregated per-date roundups, cross-checked where both cover a contest. Vote shares are quoted only where the aggregator published one; per-candidate counts are on each returning officer's page.