Rachel Reeves v the OBR

For a crew of 50-odd nerds sharing an uninspiring concrete block in Westminster with the Ministry of Justice, the Office for Budget Responsibility has come to wield extraordinary power.

- Heather Stewart

It was the OBR projections of Rachel Reeves's evaporating headroom that left the chancellor scrambling for welfare cuts in March. And it is the watchdog's rethink about productivity growth that means the chancellor is sure to deliver a second tax-raising budget on 26 November.

The Treasury is determined to avoid being dragged around by the OBR in 2026. And so, as she draws up a revenue-raising package, the chancellor is aiming to accumulate more headroom against her fiscal rules - so that every £1bn shift is less make-or-break - and to downgrade the status of the OBR's spring forecast.

Set up during the Tory-Lib Dem coalition, the independent forecaster was meant to prevent politicians from tweaking economic forecasts to suit their own ends.

As Labour came to power it embraced the new system, now 15 years old, with Reeves even legislating to prevent future fiscal events taking place without an OBR forecast, as she sought to remind voters of Liz Truss's catastrophic tenure.

Labour saw complying with the watchdog's strictures as a crucial signal to investors in government bond markets that the incoming administration had a clear plan for fixing the public finances.