Ministers face angry calls to help millions of benefit claimants who've been left out of a cash boost for coronavirus.
Universal Credit will become £1,040 a year more generous on Monday for one year to help people through the outbreak - with Tax Credits rising too.
But Jobseekers' Allowance, Income Support and Employment Support Allowance did not rise at the same rate.
Instead they will only get a long-planned 1.7% rise after years of benefit freezes - a move dubbed a "shameful two-tier welfare state".
Marc Francis of the Zacchaeus 2000 Trust said: “If £95 a week is what newly-unemployed UC claimants need to live on, then it’s surely what existing claimants need to live on as well."
Stephen Timms, chairman of the Commons Work and Pensions Committee, said it was a matter of fairness. He added: “There is a case for doing it for legacy benefits as well.”
It comes after new figures showed 880,000 people filed claims for Universal Credit in the last fortnight - around nine times the usual rate.
Some 10,000 DWP staff have been reassigned to the front line to deal with the influx as claimants say they have faced online queues of tens of thousands of people to prove their identity.
At the last count 183,000 people were on JSA, 372,000 on Income Support and 2million on ESA. Some will have since moved to UC.
Work and Pensions Secretary Therese Coffey defended the move - saying UC was chosen to "focus particularly on new claimants".
DWP Permanent Secretary Peter Schofield added the rate of legacy benefits could not be changed quickly due to older computer systems.
But Mr Francis accused him of "treating the public as fools."
He added: "The Government must now urgently revisit this shameful attempt to create a two-tier welfare state and ensure that all households who are out of work get the money they need to live on not just during this current crisis, but in the years ahead as well."
Basic rate jobseekers' allowance and ESA will each rise by £1.25 a week from Monday, to £74.35 for over-25s.
This is the equivalent of £322 a month. But the standard allowance in Universal Credit will rise from £318 to £410 for singles over 25.
ESA is being offered earlier in a person's illness as part of changes in the Budget, but not being increased significantly.
A DWP spokesperson said: “Universal Credit is delivering in these unprecedented times.
“With such a huge increase in claims there are pressures on our services, but the system is standing up well to these and our dedicated staff are working flat out to get people the support they need.
"We’re taking urgent action to boost capacity – we’ve moved 10,000 existing staff to help on the front line and we’re recruiting more.”