More than eight million households across the UK will get two cash payments totalling £650 in the months ahead to ease cost-of-living pressures.
Work and Pensions Secretary Therese Coffey set out detailed plans today.
The first instalment of £326 for qualifying low-income households in England, Wales, Scotland and Northern Ireland will land in bank accounts from July 14.
The Government said it is making "significant interventions" to support groups who are most vulnerable to rising costs.
Ms Coffey said: "With millions of the lowest-income households soon seeing the first of two cash instalments land into their bank accounts, we are taking action to directly help families with the cost of living.
"This one-off payment totalling £650 is part of our £37billion cost-of-living support package that will put an extra £1,200 into the pockets of those most in need."
To be eligible for the first instalment, people must have started a successful benefits claim by May 25.
The Government said the cash would be tax-free and not count towards someone's benefit cap.
Chancellor Rishi Sunak said: "We have a responsibility to protect those who are paying the highest price for rising inflation, and we are stepping up to help.
"I said we would stand by people when they needed help, and we are."
The second instalment of £324 will be sent to qualifying low income households in the autumn.
Consumer group Which? said the cash would "bring relief to many".
But policy director Rocio Concha told the BBC: "The success of these measures will ultimately be judged by whether financial help is getting to the most vulnerable in time to help them through this cost of living crisis."
Inflation - the rate at which prices rise - is currently at a 40-year high as the war in Ukraine and pandemic push up the cost of everyday essentials.
In May, the energy regulator Ofgem said the typical household energy bill was set to rise by £800 in October, bringing it to £2,800 a year. Bills had already risen by £700 on average in April.