The OBR's revised projections are expected to be the most pessimistic in 15 years, highlighting a significant economic slowdown despite efforts to prevent such a downgrade.
Business Secretary Peter Kyle admitted the UK is in a "growth emergency," as business leaders expressed concerns over high taxation at the CBI conference.
A House of Commons library analysis indicates the Treasury is losing up to £90bn in annual tax revenue due to the economic impact of Brexit.
Reeves's pre-Budget speech to Labour MPs reportedly focused on "fairness," cutting the cost of living, NHS waiting lists, and debt, rather than economic growth.