The Prime Minister said the Government will do "whatever it takes" to support the economy through the coronavirus outbreak.
"We must act like a wartime government and do whatever it takes to support our economy," he told a Downing Street press conference.
"We support millions of businesses and tens of millions families and individuals through coming months.
"The Government must and will act with a profound sense of urgency."
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The Prime Minister also warned that Covid-19 is so "dangerous" that without drastic action it will "overwhelm the NHS", as he warned that more "extreme measures" may be needed to protect lives in the future.
Shortly before the press conference, NHS England chief executive, Sir Simon Stevens, told MPs the NHS would be "flat out" helping coronavirus patients for the next four to six months.
He has sent a letter to hospitals telling them to prepare for a big influx of patients requiring ventilation and ordered moves to cancel non-emergency surgery by April 15 at the latest, alongside the discharge of medically fit people to free up beds.
Boris Johnson said that if the country follows scientific advice "we know that we will beat it" and that "we have the resolve and the resources to win the fight".
Support for business
Asked if individuals and businesses would lose out while the Government put the finishing touches on support measures, Mr Johnson said: "Be in no doubt that the state is asking people to do something, to make very considerable changes to their lives, and it is only right therefore that the state should stand behind people as they make those changes.
"And that's what a government, that's what a society is there to do.
"It's there to look after people through thick and thin and to make sure that we get through this together and that's what we're going to do."
The Prime Minister said the Government action represented a "huge package of support" for businesses.
"It comes in form of standing behind companies' obligations and behind their debts on a huge scale, and also making sure they have the cashflow that they need," he said.
"This is a quite extraordinary effort to put liquidity into the economy at a difficult time.
"Be in no doubt: one day the UK economy is going to bounce back and those good, brilliant British companies will bounce back, and that is why we are taking the steps to protect them that we are today."
Mr Johnson said the Government was keeping the option of closing schools under "continuous review".
Mr Johnson said: "What we've done is to ensure that where people have policies that they say that the insurers might have said only involve payout where, as it were, the insured party has been compelled to shut their business, whereas we've only urged people to stay away, and there's been some sort of quibble about whether or not to pay out, the insurers have stepped up to the plate and understood that they have to pay out to those businesses."
The Prime Minister also stated that the country “only really have a few weeks" to build the ventilators it is expected the NHS will need.
He added: "What I want, what all business wants - to explain what I'm seeing - is an end to suffering.
"That is why they are working incredibly hard in the next few weeks, and we only really have a few weeks, to build literally thousands of ventilators that this country will need."
He added: "The British industry, manufacturers are responding to this challenge with incredible energy and determination."
When asked if supermarket shelves would remain stocked, the PM responded: "We're absolutely confident that our supply chains are working and will work, and that we will get farm to fork food supplies for this country.
"Therefore, people should have no need to stockpile or to panic-buy."
Asked about schools and free school meals, Mr Johnson said: "On schools, we will have obviously plans ready to go on that.
"Gavin Williamson, Education Secretary, has a plan to make sure that parents with kids who are eligible for free school meals get the compensation or the treatment they need one way or the other, we certainly anticipated that."
Summing up the conference, he added: "What we're doing is we're asking the public to do some pretty big things in the changes we're asking us all to make in our lives and I understand the effect that that is going to have on the economy.
"I just want to repeat that key message, the more sedulously, the more zealously, the more meticulously, the more scrupulously we can follow the scientific advice that we've been given, the better we will be able to protect our NHS, the more lives we will be able to save and the quicker we will get through this experience and the more strongly our economy will bounce back.
"But, in the meantime, as you'll have gathered this afternoon, we are going to do whatever it takes."
Here's a look at the key measures the Chancellor announced during the PM’s briefing to help the economy:
- £330 billion Government-backed loans
Mr Sunak unveiled an "unprecedented package" of Government-backed loans worth £330 billion - equivalent to 15% of GDP - to help businesses.
He said: "That means any business who needs access to cash to pay their rent, their salaries, suppliers or purchase stock will be able to access a Government-backed loan or credit on attractive terms.
"And if demand is greater than the initial £330 billion I'm making available today, I will go further and provide as much capacity as required. I said whatever it takes, and I meant it."
- Mortgage holidays
The Chancellor said homeowners would get a "three-month mortgage holiday" if they are suffering difficulties due to the outbreak.
He told the press conference: "For those in difficulty due to coronavirus, mortgage lenders will now offer a three-month mortgage holiday so that people will not have to pay a penny towards their mortgage cost while they get back on their feet."
- Cash grants
Mr Sunak said he was providing cash grants of up to £25,000 to smaller businesses in the retail, hospitality and leisure sectors with a rateable value less than £51,000 to help bridge through the period.
He also said he would increase the £3,000 cash grants to 700,000 of the country's smallest businesses - announced in the Budget - to £10,000.
- Business rates holiday
The Chancellor said he was extending a business rates holiday for those smaller businesses to all businesses in those sectors irrespective of their rateable value.
It means shops, pubs, theatres, music venues and restaurants will pay no business rates for 12 months.
- People's financial security
Mr Sunak said he would soon announce further measures to "support people's financial security".
"In particular, I will work with trade unions and business groups to urgently develop new forms of employment support to help protect people's jobs and their incomes through this period," he said.