People claiming Universal Credit and other benefits will have to attend more face-to-face appointments at jobcentres from this week.
And those who fail to turn up will be sanctioned by having money cut from their payments.
In addition, the move back to in-person meetings will put them at greater risk of Covid, a union has warned.
Jobcentres fully reopened this week in line with the Government roadmap, which has also seen shops, gyms and pubs back in business.
But union leaders attacked the reopening of jobcentres, warning it risks further outbreaks of coronavirus.
The Public and Commercial Services union (PCS) said it would not rule out industrial action unless a “just settlement” can be agreed
The union said returning to pre-lockdown opening hours for jobcentres in England, Wales and Scotland will lead to a huge increase in face-to-face appointments for people claiming Universal Credit and other benefits.
Work Capability Assessments are carried out for those on Universal Credit or Employment & Support Allowance.
Assessments are also needed for Personal Independence Payment (PIP) and Industrial Injuries Disablement Benefit.
Such appointments have been taking place by phone or written forms since March 2020 but are to start resuming in person.
The move threatens “unnecessarily risks”, especially as staff were delivering services to claimants successfully while working from home, said the union.
PCS general secretary Mark Serwotka said: “This reckless move by ministers is wholly unnecessary and risks putting both claimants and jobcentre staff in harm’s way.
“Staff have been doing an incredible job delivering key services such as Universal Credit and helping those most in need, access the assistance they require, throughout the pandemic.
“It is counterproductive and arrogant for ministers to risk staff and the wider public’s health by resuming normal jobcentre opening hours before the vaccine is fully rolled out and when these services are being successfully delivered from home.
“The anger of our members is palpable and we are not ruling out strike action until a just settlement is found.”
A Department for Work and Pensions spokesman said: “Throughout this pandemic, jobcentres have remained open to ensure we can continue to provide vital support to the most vulnerable.
“Our return to full opening hours will enable us to provide even more help and support to those who need us.
“We take the health and safety of colleagues extremely seriously, and are absolutely committed to ensuring all our sites remain Covid secure in line with Public Health England and Government guidance to keep colleagues and customers safe.”
It has produced guidelines for attending assessments including wearing face masks and practising social distancing.