Britain said it would offer £330 billion pounds of loan guarantees, equal to 15% of its gross domestic product, and other measures to help struggling business as it ramped up its attempt to fight the economic hit from coronavirus.
"This is a time to be bold, a time for courage," finance minister Rishi Sunak said, speaking alongside Prime Minister Boris Johnson.
Sunak said he was ready to increase the size of the loan guarantees to ensure cash gets to all companies needing it as their businesses slump.
Other measures announced included a three-month mortgage holiday for people in difficulty and an extension of a suspension of a property tax paid by small companies to all businesses in the hospitality and leisure sectors.
Those companies would also get a cash grant and the government would discuss a support package for airlines and airports, Sunak said.
"This struggle will not be overcome by a single package of measures or isolated interventions," he said.
"This national effort will be underpinned by government interventions in the economy on a scale unimaginable only a few weeks ago."
Johnson on Monday told people to avoid pubs, clubs, restaurants, cinemas and theatres, leaving many companies facing the prospect of collapse.
Earlier on Tuesday, Britain's budget forecasters said the scale of the borrowing needed to fight the coronavirus crisis in the economy might resemble the country's immense debt splurge during World War Two.