The Organisation for Economic Co-operation and Development (OECD) says the coronavirus pandemic risks worsening child poverty and inequality in the UK.
Its latest report on the country says it may also exacerbate weak productivity and regional disparities.
The think tank warns another major negative impact on trade and jobs could come from a no-deal Brexit.
The UK faced “prolonged” disruption and was at a critical juncture, it said.
The think tank did, however, revise its estimate for the UK economy and now expects it to contract by 10.1% rather than the 11.5% it forecast in June.
The OECD, which is a economic organisation with 37 member countries, said the UK’s strict lockdown was essential to contain the pandemic but halted activity in many key sectors.
It said: “While restrictions have eased, the country now faces a prolonged period of disruption to activity and jobs, which risks exacerbating pre-existing weak productivity growth, inequalities, child poverty and regional disparities.”
It said any new measures to control the pandemic needed to be “carefully calibrated” to manage the economic impact.
It added that leaving the EU Single Market in January could also create economic challenges, noting how deeply integrated UK and EU had become.