Devolved leaders will stress the need for unfettered access to the European Single Market at a summit meeting with UK government ministers set to be dominated by Brexit.
Ahead of the British Irish Council (BIC) summit near Cardiff, Welsh First Minister Carwyn Jones called for firm commitments on free trade.
Mr Jones, who is hosting the event, said single market access had become even more important amid the prospect of protectionist policies in a Donald Trump White House.
"This is a changing world and these are tumultuous times," he said.
"But our focus on retaining full and unfettered access to the single market is unwavering.
"Today I will once again call on the UK Government to make this their absolute priority in forthcoming Brexit negotiations.
"Let me be clear, a situation where the UK faces barriers and tariffs to trading with the European market would be disastrous for our economy and I will not support any settlement that undermines our exports."
Mr Jones will be joined by Scottish First Minister Nicola Sturgeon, Northern Ireland's First Minister Arlene Foster and Deputy First Minister Martin McGuinness and the premier of the Irish Republic, Taoiseach Enda Kenny, at the BIC summit.
Welsh Secretary Alun Cairns, Northern Ireland Secretary James Brokenshire and Parliamentary Under Secretary at the Department for Exiting the European Union Robin Walker will be among UK Government representatives attending.
Ahead of the event, the Scottish Government said the UK must not be "driven-off a hard Brexit cliff edge".
External Affairs Secretary Fiona Hyslop said: "A hard Brexit outside the Single Market, is the biggest threat to the economic prosperity of Scotland and all the UK.
"Despite repeated calls from the Scottish Government and other administrations who are understandably equally concerned, the UK Government is still failing to provide any transparency on their plans with only a few weeks of 2016 left.
"There is currently no confidence in the UK Government's ability to fight for the benefits that the single market brings, including free movement of people, and all signs appear to be pointing to the fact they will choose to throw these away.
"We must not be forced into a race to the bottom in terms of standards and employment protection."
Commenting on the potential implications of the US presidential election, Mr Jones added: "President-elect Trump was elected on the promise to make America Great Again.
"He has made no secret of his overriding priority to get the best deal for America and I am totally unconvinced we will see a free trade deal that will do anything other than benefit the US.
"Indeed, access to the US market may become more, not less difficult. It is therefore even more important to secure the quality of our access to the European market of 500 million people.
"We stand at a major crossroads for Wales and the United Kingdom, and decisions taken now will determine our future for decades ahead."
The BIC was established as part of the 1998 Good Friday peace agreement in Northern Ireland.
Its aim is to promote links between the UK's devolved regions, the Irish Republic, the Isle of Man, Jersey and Guernsey.
Its membership comprises representatives from the Irish Government; UK Government; Scottish Government; Northern Ireland Executive; Welsh Government; Isle of Man Government; Government of Jersey and Government of Guernsey.
The chief ministers of the Isle of Man, Jersey and Guernsey - Howard Quayle, Ian Gorst and Gavin St Pier – will attend Friday's summit along with ministerial colleagues from their respective administrations.
Mr Kenny and Ms Sturgeon will use the summit to hold separate bilateral talks, a spokesman for the Dublin government said.
During the main discussions, the Taoiseach is expected to give an overview of Ireland's preparedness for Brexit as well as a briefing on Ireland's all-island Brexit forum - which has been snubbed by unionists - and the outcome of the North South Ministerial Council.
Ahead of the summit, a spokeswoman for the UK Government said: "We are determined to make sure that the UK remains the best place in Europe to run and grow a business.
"The UK will continue to be a global leader of free trade and we want to see British companies having the maximum freedom to trade with and operate in the Single Market.
"The Government will continue to work closely with British businesses as we approach the negotiations so that together we can make a success out of Brexit.”