"And the risk now is that renewed containment measures send the recovery into reverse," she added.
Households saved a record 29.1 percent of their income, up from 9.6 percent in the first quarter, as they were unable to spend in many shops and restaurants during the lockdown, while incomes were supported by a government job program which ends next month.
Britain has suffered Europe's highest death toll from Covid-19, with more than 42,000 fatalities.
Compared with a year earlier, Britain's second-quarter output tumbled 21.5 percent — the same as in Spain — while France reported a 19.0 percent drop.
The statistics office said differences in how countries estimated public sector activity — especially whether they focused on money spent or used the ONS' approach of looking at the extent of disruption to normal services — complicated international comparisons.
Nonetheless, it said the UK economy shrank more than any other Group of Seven economy in the first half of 2020.
There have been some bright spots in the recovery.