LONDON, June 17 (Reuters) – Britain’s economy has seen weak growth overall since it left the European Union at the start of 2020, though disentangling the effects of Brexit from the COVID-19 pandemic which hit Europe weeks later has been hard for analysts.

Following is a summary of estimates from official bodies and other major researchers.

U.S. National Bureau of Economic Research (November 2025)

Estimated Impacts on UK Economy

• Brexit reduced UK GDP by 6%-8% by 2025 compared with if Britain had remained in EU

• Productivity and employment reduced by 3%-4%

• Investment reduced by 12%-18%

Underlying Causes and Methodology

• Weakness caused by greater business uncertainty hitting investment, lower expected demand and slower productivity growth due to distraction of managing Brexit and greater impact on more productive firms that traded internationally

• Calculations based partly on a ‘synthetic’ counterfactual UK based 61% on the United States, 11% on Estonia, 10% Greece and 7% Italy, as well as other countries which together matched Britain’s pre-Brexit economic performance

• Research conducted by economists affiliated to Stanford University, the Bank of England, the Deutsche Bundesbank, King’s College London and the University of Nottingham

Julian Jessop, Institute of Economic Affairs (November 2025)

Critique of NBER Methodology

• Criticises NBER methodology for heavy weighting given to U.S economic performance, assumption that countries that UK matched pre-Brexit are a good post-Brexit match

• U.S. growth has been an outlier since 2020, UK GDP per capita growth similar to Germany and France

• 8% higher GDP would require UK to have significantly outperformed other big European economies

• Substantially better UK employment performance within EU unlikely given already low unemployment

• Brexit uncertainty a temporary hit to investment, not lasting

UK Office for Budget Responsibility (July 2025)

Long-Term Productivity and Trade Effects

• Post-Brexit trading relationship to reduce long-run productivity by 4% relative to staying in the EU

• Two-fifths of this impact had occurred before a post-Brexit trade deal came into force at the start of 2021

• Britain’s EU exports and imports will be 15% lower in the long run than if it had remained

• New trade deals with non-EU countries will not have a material impact

• Calculations based on analysis of historic trade deals

UK National Institute of Economic and Social Research (April 2025)

GDP, Productivity, and Investment Losses

• 2%-3% loss in GDP per capita and labour productivity by 2023, rising to 5%-6% by 2035

• 12%-13% decline in business investment by 2023, reducing to 7%-8% by 2035

Modeling and Economic Mechanisms

• Brexit impacts modelled as a decline in trade, permanent increase in uncertainty and reduced productivity, fed into NIESR’s standard model of the economy

• Increased trading costs lead to fewer high-productivity UK firms exporting while reduced competition from the EU leads to more low-productivity firms serving the domestic market