The Government is going to cover 80% of the wages of workers up to a total of £2,500 a month under unprecedented intervention in a bid to save jobs.
The Chancellor conceded that people had already lost their jobs because of the coronavirus crisis, but he unveiled a series of measures aimed at averting an employment catastrophe in the coming weeks.
A Coronavirus Job Retention Scheme has been set up, available for any employer in the country, covering small or large companies, charities or non-profit organisations.
Rishi Sunak said: "Employers will be able to contact HMRC for a grant to cover most of the wages of people who are not working but are furloughed and kept on payroll, rather than being laid off.
"And, of course, employers can top up salaries further if they choose to.
"That means workers in any part of the UK can retain their job, even if their employer cannot afford to pay them, and be paid at least 80% of their salary."
The scheme will cover the cost of wages backdated to March 1 and will be open initially for at least three months, with the Chancellor pledging to extend for longer if necessary.
"I am placing no limit on the amount of funding available for the scheme. We will pay grants to support as many jobs as necessary.
"To meet our commitment to that effort, I am today announcing a combination of measures unprecedented for a government of this nation."
The Chancellor said HMRC was working night and day to get the scheme up and running and he expected the first grants to be paid within weeks, adding: "We're aiming to get it done before the end of April."
The Government is launching a major national advertising campaign in the next few days to communicate the available support for businesses and people.
In an appeal to businesses, the Chancellor said: "Please look very carefully at that support before making decisions to lay people off.
"We are starting a great national effort to protect jobs, but the truth is we are already seeing job losses, and there may be more to come.
"I cannot promise you that no one will face hardship in the weeks ahead."
Unions have welcomed the measures, described by Unite leader Len McCluskey as "historic, bold and very much necessary".
Other union leaders warned that workers had already lost their jobs, and others will suffer hardship, even on 80% of their wages.
Paddy Lillis, general secretary of the shopworkers' union Usdaw, said: "There are many workers, particularly in retail, who are contracted for fairly few hours each week, but regularly work many more to make a weekly wage they can live on.
"These short-hours contract workers rely on this regular additional money, so for their income to be drastically reduced to 80% of contract pay will put them in real hardship."