The head of Britain's big business lobby on Monday urged companies to shift to "an emergency footing" as the prospect of a no-deal Brexit becomes more likely.
The Confederation of British Industry in a report also warned that the EU was less prepared than Britain for such an eventuality but said both sides were not ready.
CBI chief Carolyn Fairbairn wrote in The Times that a no-deal Brexit had become a "serious possibility".
"Now is the time to shift to an emergency footing.
"While it's not possible to seal our economy from all the damage the floodwaters of no deal will cause, we can lay down the sandbags and protect as much as we can," she said.
The CBI's head of EU negotiations Nicole Sykes, however, tweeted that it was "like putting sandbags down for a flood.
"Your kitchen's still going to be underwater but MAYBE we can save the bedroom upstairs," she wrote.
The CBI report said: "The EU lags behind the UK in seeking to prevent the worst effects of a no-deal scenario.
"Although businesses have already spent billions on contingency planning for no-deal, they remain hampered by unclear advice, timelines, cost and complexity," it said.
The report noted that larger companies in highly regulated areas such as financial services had prepared contingency plans but preparations in other sectors such as agriculture and manufacturing were lagging behind.
"There's a great deal more that the government, and firms, must do," Fairbairn said, calling in particular for a government information campaign to target smaller businesses that need the most help to prepare.
But she concluded that even with preparations in place "the shock of no deal can only be reduced, not removed. It's a shock that will reverberate for years to come".