Theresa May must ask the EU to delay Brexit, the Welsh and Scottish first ministers have said in a statement.
Mark Drakeford and Nicola Sturgeon said an extension to Article 50, the process for leaving the EU, was needed.
They made the call as Mrs May headed to Brussels to seek legal changes to the withdrawal deal she signed with the EU.
Welsh Secretary Alun Cairns said she was "within spitting distance of landing a deal - providing the European Union are prepared to move".
In a joint statement, the Welsh and Scottish First Ministers said: "The point has been reached where there is now no time to waste.
"We therefore renew our call for the prime minister to make clear that she and her government will ensure 'no deal' is taken off the table."
They said: "This should include putting forward secondary legislation now to remove 29 March 2019 as Exit Day from the EU (Withdrawal) Act.
"The prime minister must also request an extension from the EU of the Article 50 deadline."
"We call on the prime minister to request such an extension immediately to put an end to the threat of the UK crashing out of the EU without a deal in only eight weeks' time," they added.
Mrs May is due to arrive in Brussels later on Thursday to seek legal changes to the withdrawal deal she signed with the EU, hoping these changes will help her get her deal through the UK Parliament.
Earlier this week, Mrs May was on a two-day visit to Northern Ireland try to reassure people she can secure a Brexit deal that avoids a hard border with the Republic of Ireland.
On Wednesday, Welsh Secretary Mr Cairns said the UK government was "within spitting distance of landing a deal - providing the European Union are prepared to move".
"If the EU are prepared to move in the same way that we've moved on many areas, I believe that we can get to that position," he said.