Britain exited the EU on Jan. 31, 2020, though remained in the bloc's single market and customs union for 11 more months.
The government, led by Brexit-supporting Prime Minister Rishi Sunak, says Britain is prospering with new-found freedoms.
"Our problems pre-date Brexit," Lyons said, pointing to chronically low rates of investment in Britain.
Exports, especially in goods, have disappointed over the last three years - despite high hopes for a "Global Britain" rebalancing of the economy after Brexit.
Britain still boasts higher rates of employment and lower unemployment than most EU countries but there are some signs that Brexit may have impacted the labour market too.