Office staff will be given a "work from home" order within a fortnight if the "rule of six" fails to bring down coronavirus infection rates, ministers have been warned.
The current shortage of Covid-19 tests means employers will have no choice but to send more workers home, undermining the already weak economic recovery, business leaders said.
Senior Government sources said it would take two weeks to assess whether the "rule of six" had brought down infections. If it was found that it had failed to do so, further lockdown measures may be required.
Retailers have warned that pubs, cafes and restaurants that depend on office workers for their trade will go out of business in increasing numbers if there is not a widespread return to office working.
The number of people returning to their workplaces (see video below) has been rising since schools were reopened at the start of the month, but that trend is likely to go into reverse if the "rule of six" does not bring the infection rate down without being able to mitigate it with mass testing.
On Wednesday another 3,991 people tested positive for coronavirus, double the daily number at the start of the month. Cases are now rising as steeply as they did at the start of the pandemic, and the Government appears to have softened its "back to the office" rhetoric as a result.
The Prime Minister warned on Wednesday night that further curfew measures such as shutting pubs early may necessary to bring down infections. He told The Sun an “awful lot of people” could still die if Covid is allowed to “rip” through the country again.
Referring to curfews, which would mean early closing for pubs and restaurants, Boris Johnson said: “I remember when the pubs used to close at 11 anyway in the old days. That sort of thing, we will be looking at it.”
Mr Johnson's advice to "go to work if you can" has been dropped from official messaging, and Downing Street insisted there was no official "back to work" campaign.
A spokesman said decisions on whether people should go to the office were entirely for employers and their employees.
But with Mr Johnson admitting on Wednesday that demand for tests is outstripping supply (see the graphic below for details of how the UK's testing system breaks down) and the Health Secretary, Matt Hancock, warning that it might be weeks before the problem is solved, business leaders said employers would have no choice but to tell more staff to work from home if cases kept rising.
Matthew Fell, the UK chief policy director of the CBI, said: "If we are to successfully encourage more people into their workplace safely, then the test and trace system will be a key component. Reports of people being unable to access tests in their area or waiting too long to get the results will be deeply frustrating for individuals and businesses alike.
“No one doubts how much effort is going in to get it right. The prize is having a faster turnaround in getting results which will let people know where they stand and whether they need to self-isolate or can return to work quickly."
Adam Marshall, the director general of the British Chambers of Commerce, said: "A truly comprehensive test and trace programme is essential if the UK is to manage the virus without further lockdowns, which will cripple businesses. Continuing delays and a shortage of tests saps business, staff and consumer confidence at a fragile moment for the economy."
Meanwhile Professor Kevin Fenton, the London director of Public Health England, suggested curfews could soon be imposed across the capital if cases surged.
He added that "we're beginning to see more cases and clusters occurring in workplaces" but could not be sure whether people had caught the virus at work or in the community.
Government experts including Sir Patrick Vallance, the chief scientific adviser, have in the past urged caution from ministers about a return to office working.
Other academics have cited data showing that London, where fewer people have returned to their workplaces, has a lower infection rate than more industrialised cities in which more workers have gone back, despite the capital's much higher population density.