Britain needs to stay inside the single market and customs union until a final deal with the EU is in force, a leading business group has warned.
The CBI said even with the greatest possible goodwill on both sides of the Brexit talks, it is "impossible" to imagine the details of a new trade deal will be clear by the time the Article 50 process ends on March 29 2019.
CBI director-general Carolyn Fairbairn urged negotiators to agree transitional agreements as soon as possible, in order to reduce uncertainty and protect jobs.
"Instead of a cliff edge, the UK needs a bridge to the new EU deal," Ms Fairbairn said in a lecture at the London School of Economics.
"Our proposal is for the UK to seek to stay in the single market and a customs union until a final deal is in force. This would create a bridge to the new trading arrangement that, for businesses, feels like the road they are on."
Ms Fairbairn also highlighted the importance of firms making only one transition, rather than forcing them to adapt twice - first to the transition, and then to the final plan.
She said: "Making two transitions, from where firms are now to a staging post and then again to a final deal, would be wasteful, difficult and uncertain in itself.
"One transition is better than two and certainty is better than uncertainty."
On the deal itself, Ms Fairbairn echoed the language of Brexit Secretary David Davis who has promised an agreement "like no other in history".
"We're seeking the most ambitious and comprehensive free trade deal ever agreed in history," she said.
But she warned the prospect of a damaging cliff edge is casting "a long shadow" over businesses, resulting in investment decisions being deferred or lost.
She said: " Agreement is needed fast - waiting until March 2019 is too late.
"The exact mechanisms to achieve this can be debated and negotiated. But for businesses making daily decisions about where and how much to invest, this is the simplest answer to the uncertainty they face today."
Ms Fairbairn will attend a conference of business leaders on Friday, hosted by the Brexit Secretary at Chevening House in Kent, as part of a series of events to "intensify" government engagement with industry.
It comes after reports that corporate leaders wanted ministers to consult them more on Brexit and amid suggestions of a power struggle in the Cabinet over the shape of the UK's exit from the EU.
Prime Minister Theresa May said: "What we want to do of course is ensure that at the end of the negotiating period we've got not just the arrangements for our exit from the European Union but also the future relationship with the European Union.
"We're going in to negotiate a comprehensive free trade agreement so we can continue to see that trade - we want to see tariff-free trade, we want to see as frictionless border as possible.
"We will then be able to discuss what I sent out in my Article 50 letter which is implementation period because of course what we want to ensure is that that process of not just withdrawing but moving to a new relationship is as smooth and orderly as possible and that means ensuring that where there are practical reasons why you need to have an implementation period - those will be discussed, those can be agreed and people can put in place the necessary practical steps that they need."