Jobseekers risk losing their benefits if they refuse to look for work using a new government website, it has been announced.
Until now it has been voluntary to seek work via Universal Jobmatch, an automated system for matching jobs to skills and training.
The Department for Work and Pensions said job advisors will now require Jobseeker's Allowance claimants to use the service, taking into account their access to the internet.
If a jobseeker refuses without good reason, they risk losing their benefits.
Employment Minister Mark Hoban said: "Jobseekers who are serious about looking for work will get all the support they need from us. But in return for receiving Jobseeker's Allowance they must prove they are doing everything they can to find a job.
"Universal Jobmatch has already proved a big success in getting people off benefits and into work, and people are flocking to use it. If someone refuses to use a service that already has 460,000 employers posting jobs, you have to ask why - which is why we are now saying claimants must use it."
Universal Jobmatch was launched last year and is open to all jobseekers, even those not claiming a benefit.
A Public and Commercial Services union spokesman said: "We have very serious concerns about how robust and secure Universal Jobmatch is, particularly given the number of rogue job adverts that have appeared, which DWP shockingly says it is powerless to prevent.
"Using technology to help jobseekers is sensible but making it possible to force people to use this flawed system is just another crude tool in the Government's cruel approach to welfare that seeks to blame unemployed people for a chronic lack of jobs.
"We have issued very clear guidance to our jobcentre members that their managers have no flexibility to interpret the rules and advisers should resist any attempts to set targets for making Universal Jobmatch mandatory. This guidance also confirms that jobseekers do not have to give DWP access to their accounts."