Jenny Bates, a Friends of the Earth clean air campaigner, said: "Seeing this drop in air pollution shows that less traffic can quickly lead to cleaner air.
"Once this dreadful situation is over, we don't want to rush to go back to where we were or worse, and we can't have an accelerated return to business as usual.
"We can have a better, cleaner future for ourselves and the planet."
A week later, Mr Johnson announced a nationwide lockdown in a bid to stop the spread of the virus.
Daily readings from air quality monitoring stations since 17 March were compared to the same period last year, accounting for weekdays.
Analysis by the BBC Shared Data Unit found air pollution has halved at some of the most polluted sites this time last year, including readings in London, Glasgow, Bristol and Oxford.
Hafodrynys Street in Wales is known for having some of the highest levels of nitrogen dioxide outside of central London.
William Bloss, a professor of atmospheric science at the University of Birmingham, said: "What we're seeing in the lockdown... is the reductions in road traffic in our cities translating into much lower levels of NO2.
"We're seeing the reductions are greatest in areas most heavily influenced by road traffic, so city centres, roads in London, Birmingham and other urban centres."