Sanctions on benefits will return on Tuesday after a three-month freeze, as job centres re-open since the lockdown.
In March, the government said people on benefits and Universal Credit would not be penalised for 90 days if they failed to look for work, due to coronavirus.
Labour has called for an extension to the ban, saying there is an “unemployment crisis looming”.
But Work and Pension Secretary Therese Coffey said the return of “claimant commitment” rules was “essential”.
Over 5.2m people claimed Universal Credit in May, compared to just over 2m in May 2019 – although some of the rise may be covered by the phased roll-out of the system and they will not all be jobseekers.
The government had paused the requirement for claimants to actively look for work, or be available to work. Face-to-face appointments in job centres were also halted, as well as reviews and reassessments for a number of benefits, including Universal Credit and Personal Independence Payments (PIP).
During questions in the Commons, shadow work and pensions secretary, Jonathan Reynolds said the plan to re-introduce sanctions – which could lead to benefit cuts for some – came at a time when “unemployment has risen sharply”.
“Where vacancies have dropped, when people are shielding and the schools haven’t yet gone back, threatening people with reducing their support if they don’t look for jobs is surely untenable”, he said.
He called on Ms Coffey to announced an “an immediate extension”.
But the minister said: “It is important that as the job centres fully re-open this week that we do reinstate the need for having the claimant commitment and it is an essential part of the contract to help people start to reconsider what vacancies there may be.”
After the exchange, Mr Reynolds released a statement, saying the decision was “incomprehensible” and that job centres were “lacking guidance” on how to re-open safely.
He added: “With the unemployment crisis looming, it is alarming that there is no thought being given on how to offer proper support to those seeking work at this time.
“We need a proper plan from the government to get Britain back to work – sanctions aren’t the answer.”
The BBC has contacted the Department for Work and Pensions for a response.