Measures introduced since Boris Johnson came to power, including Wednesday’s Budget, will cost households £3,000 more tax a year, according to the Resolution Foundation.
The think tank found policies announced by Chancellor Rishi Sunak had boosted incomes by 2.8% for the poorest fifth of households.
However, households on middle incomes would take a 2% hit, it added.
Mr Sunak maintained his Budget had “cut taxes for millions of the lowest-paid”.
That claim was challenged by the Foundation’s figures.
It said the changes to the amount families can keep from their earnings if they are on universal credit only partially offset the effect of withdrawing the £20-a-week boost during the pandemic.
On average, recipients would lose £800 a year, it calculated.
The figures produced by the Resolution Foundation include business taxes, which are regularly passed on to consumers.
Of the 4.4 million households on universal credit, about three-quarters (3.2 million households) will be worse off as a result of decisions to take away the £20-a week-uplift, despite the chancellor’s new universal credit measures.
But 1.2 million households would be better off by £900 a year than before the Budget.