The UK is unlikely to get a full trade deal with the EU before it leaves the union, the first minister has warned.
Carwyn Jones told AMs "transitional arrangements" would be needed to avoid trade "falling off a cliff".
The prime minister has said the formal Brexit negotiation process will begin by the end of March, with the UK set to leave the EU by summer 2019.
On Friday, Mr Jones said putting new trade arrangements in place within that timescale would be a "world record".
Giving evidence to the assembly's committee for the scrutiny of the first minister, he said: "I think it is hugely difficult to imagine a scenario where within two-years-and-a-bit there is a comprehensive arrangement between the UK and the EU."
"I think that would be a world record in terms of getting that done in that amount of time.
"The UK government needs to look at what needs to be done to bridge any gap that might be between the ending of the Article 50 process and a more lasting, sustainable settlement to avoid going off the edge of a cliff and then having to climb back up it when a lasting solution is found."
Mr Jones added that there was a "need to think about what the transitional arrangements might be whilst respecting, of course, the results of the referendum."
He also called for a new independent body to adjudicate disputes between the devolved governments and the UK Treasury.
In the event of a disagreement between Whitehall and Cardiff, Edinburgh or Belfast on how financial decisions should be made following Brexit, Mr Jones said: "The UK will have to develop its structure as we leave the EU.
"We can't go to a situation where decisions on the UK's internal market, or financial decisions, are governed entirely by Whitehall - there has to be agreement between governments."
Meanwhile the Supreme Court has said it will allow the Scottish and Welsh governments to intervene in the UK government's appeal against a ruling that Parliament must be consulted on the triggering of the formal Brexit process.
In Friday's scrutiny session in Cardiff Bay, Mr Jones also said UK ministers needed to take "more of an interest" in the steel industry to secure its future.
Commenting on the current position at Tata's Port Talbot plant, the first minister said there had been less "engagement" with the firm under Theresa May's leadership than there there had been under her predecessor David Cameron.
Downing Street has been asked to respond.