The Resolution Foundation said fewer than 4.5 million workers were currently furloughed, as employers begin contributions toward the scheme.
It comes as some businesses face putting workers back on furlough after lockdown easing was halted in England.
The government said millions of jobs had been saved because of the furlough scheme.
Latest furlough figures showed 9.5 million workers as being on the scheme, at a cost of £31.7bn to the Treasury.
But the Resolution Foundation said that, although more than nine million workers - a third of the private sector work force - have been furloughed at some point since March, the current number is now less than half that.
The think tank warned winding the scheme down "carries the real risk of increased redundancies", echoing concerns raised earlier this week that it could push unemployment to 10% this year.
Firms will now have to pay towards the cost of furloughed staff, by paying their employer National Insurance and pension contributions until the scheme ends on 31 October.
The average cost will be £70 a month - or 5% of the employees' pre-furloughed pay - according to the think tank.
But many businesses in England expecting to reopen this weekend heard during a Downing Street press conference on Friday they would have to wait at least another fortnight because of a rising number of coronavirus cases.
Casinos, bowling alleys, skating rinks and close-contact beauty treatments are among those to be affected by the latest changes.
The number of furloughed workers at any one time peaked at nearly eight million in late April, the foundation said, based on analysis of three separate Office for National Statistics surveys.
Dan Tomlinson, senior economist at the Resolution Foundation, said: "The Job Retention Scheme has supported around a third of the private sector workforce at some point since lockdown began, protecting family incomes and preventing catastrophic levels of unemployment."
But he said that millions of employees are currently without work, particularly in sectors such as hospitality and leisure, and called for the government to phase out support for these "hardest hit" sectors more slowly, due to a "heightened" risk of unemployment.
A Treasury spokesman said: "We said at the start of the crisis that we couldn't save every job - but it's clear that the furlough scheme has saved millions of them - and now many people who've been furloughed are able to return to work.
"That's good for the economy but more importantly it's good for individuals, their families and communities."