The cabinet has decided to "ramp up" preparations for a no-deal Brexit amid uncertainty over the fate of Theresa May's proposed EU exit deal.
It has approved £2bn to help if the PM's deal is rejected and the UK leaves on 29 March without an overall deal.
Letters will be sent to 140,000 firms updating them on what they should do while 3,500 troops will be put on standby to help government departments.
Lib Dem leader Sir Vince Cable said it amounted to "psychological warfare".
With 101 days left until Brexit and many MPs still opposed to the government's withdrawal agreement, which MPs will vote on in mid-January, ministers met for a longer-than-normal two and a half hour meeting.
They agreed that businesses should activate their own 'no deal' contingency plans, as they think appropriate.
Updated Revenue and Customs information packs will be sent to firms later this week, setting out what changes could be needed at the border.
Consumers are being advised to familiarise themselves with advice published this summer, in areas ranging from booking flights to using credit cards, with more details promised in the coming weeks.
Separately, Defence Secretary Gavin Williamson has told MPs that 3,500 military personnel, including logisticians and engineers as well as infantry units, were ready to be deployed if needed.
About 10% of the force would be reservists who will receive their call up papers in the middle of January so that if needed they would be ready in March.
Brexit Secretary Stephen Barclay said the cabinet had agreed that "preparing for a no deal will be an operational priority within government but our overall priority is to secure a deal".
He said no-deal planning "needs to be much more of a priority for businesses" and there would be a "significant increase" in the guidance issued to them over the next 14 weeks, as Brexit day approaches.
E-mails will be sent out to 80,000 of those businesses most like to be affected over the next few days.
In the autumn of 2017, The Treasury earmarked £3bn for 'no deal' planning.
In March, Chancellor Philip Hammond said half of that had been allocated to 20 government departments, with the Home Office, transport, the environment and business among the largest recipients.
At Tuesday's cabinet meeting ministers approved £2bn to go to departments for the 2019/20 year, with the priority areas being borders, security and international trade.
Health Secretary Matt Hancock has already ordered full "no deal" planning across the National Health Service, he told the BBC's Newsnight on Monday.