Chancellor Rishi Sunak also made billions of pounds available to help firms affected as he said: "We have never in peacetime faced an economic fight like this one."
Saying it is a "time to be bold, a time for courage", he said: "We will support jobs, we will support incomes, we will support businesses, and help protect loved ones.
"We will do whatever it takes."
Any business in need of cash to pay rent, staff, or buy stock will be able to get a loan "on attractive terms", Mr Sunak said as he announced £330 billion of Government-backed and guaranteed loans, which he said is worth around 15 per cent of the UK's GDP.
"If demand is greater, I will go further and provide as much capacity as required", he said.
More help is due to be announced in the coming days, specifically for airlines and airports, while talks will be held with trade unions and business leaders.
After concern was voiced about the impact of Boris Johnson's social distancing plea, with people urged to avoid pubs, clubs, theatres, and cinemas, Mr Sunak said firms in the retail, hospitality, and leisure industry will pay no business rates for 12 months and will be able to apply for grants if they are not covered by pandemic insurance too.
Mr Johnson defended his drastic measures announced yesterday - including asking people to work from home, avoiding all needless social contact, and telling society's most vulnerable people to stay inside for 12 weeks from this weekend - and warned more are on the way.
He told a press conference: "We must stop the disease spreading to the point where it overwhelms the NHS."
He said Covid-19 is "so dangerous and infectious" and said that, without "drastic measures it would overwhelm any health system in the world".
"The measures set out yesterday will have an effect, while the shielding of the vulnerable will also help," he said during a second daily briefing.
"They are extreme but we may have to go further and faster in the coming days.
"Yes, this enemy can be deadly, but it is also beatable and we know how to beat it. We know if, as a country, we follow the best scientific advice, we know we will beat it.
"However tough the months ahead, we have the resolve and the resources. This Government will do whatever it takes."
Mr Johnson said thousands of officials are "working round the clock", and insisted Whitehall is "doing what we can to increase the capacity of the NHS" through "more testing, more beds, more staff, and much better data and technology".
Saying we must study Covid-19 and see what helps treat severe cases, he also said: "We must act like any wartime government and do whatever it takes to support our economy."