Carolyn Fairbairn, director-general of the CBI, told the Yorkshire Post that a "reckless" no-deal Brexit would be "extraordinarily challenging" for most British businesses.
When asked about how CBI members feel about Brexit, she said: "I think the honest turn of phrase would be verging on despair actually.
"It is unbelievable to them that here we are, 16 days from the date of leaving the European Union with no more clarity than we had on June 24, 2016, the day after we had the referendum.
"That is shocking. They regard it as a failure of political leadership, but they are also practical and the vast majority of businesses, 60 per cent, have implemented contingency plans for a no-deal. Having said that, only 4 per cent feel they are fully prepared because you can't be.
"That's because you can't prepare for no deal. You can mitigate, you can limit, but you can't take away the cost of the impact," she said.
"The business community is despairing, frustrated, incredulous, but practical, and getting on with it."
Talking about the dangers of a no-deal Brexit, Ms Fairbairn said CBI members can see "absolutely no advantages at all" of WTO rules.
"WTO terms are the most rudimentary trade terms in the entire world. There is no country in the world that trades on WTO terms alone. We would be joining a club of one," she said.
"The frictions that would be created in our trading system are unconscionable really. 90 per cent of our exports would face tariffs. We would be introducing customs checks at borders, five times the volume of customs checks compared with now. Our systems are not ready.
"So WTO terms - I've used the term reckless for people who have tried to say that it would be easy. There is not a shred of evidence from anybody that this would be anything other than an extraordinarily challenging thing for the majority of British businesses."
Ms Fairbairn also criticised MPs and commentators who have advocated extremes in the Brexit debate.
"Ideology has trumped evidence too much in this debate," she said.
"If you want to believe something hard enough, you just don't listen to what the facts are.
"We've had four quarters of declining business investment in this country, we've had £900bn of financial capital from our banking system moved out of the country."
Ms Fairbairn said the UK can expect nothing more from Brussels.
"It's the end of the road. There is no more rope that we can expect to have from Brussels," she said.
"A number of things now need to happen. First and foremost there needs to be compromise. That means a revisiting of red lines.
"Having looked at all the analysis, the benefits of a customs union for the UK significantly outweigh the costs.
"We will also be saying there needs to be a process whereby that kind of consensus might be allowed to materialise because an extension without a plan is not good for anybody."
Given a choice, she said CBI members will prefer a delay to no-deal.
"That is absolutely categoric, but they would like that delay to be as short as possible - as long as necessary, but as short as possible," she said.
"Every week of delay is another week of cost if you are a business that is having to create buffer stocks or hire new warehousing. There has to be a plan."
Asked for her take on calls for a People's Vote, she said: "The People's Vote opens up another long period of serious uncertainty and we know what the impact that has already had on investment and jobs and that is our number one concern - the protection of investment and jobs.
"Having said that, if it becomes the only way of breaking the gridlock, the only way of getting out of this interminable lack of decision, then business would engage with it."