The UK constitution is "no longer fit for purpose", Labour leaders have warned, as the party prepares to convene its new devolution taskforce.
The group, set up by Labour to examine how to redistribute powers across the UK, will meet for the first time on the day the Prime Minister formally triggers Brexit and less than 24 hours after the Scottish Parliament voted in favour of a second independence referendum.
The taskforce, which includes former prime minister Gordon Brown, Welsh First Minister Carwyn Jones, Scottish Labour leader Kezia Dugdale and former deputy prime minister John Prescott, believes the only way to "unite our deeply divided kingdom" is by handing more powers to the nations and regions that make up the UK.
Mr Brown has proposed the creation of a "Council of the North", which would complement George Osborne's Northern Powerhouse initiative and co-ordinate the region's economic activity.
Mr Jones will host the first meeting in Cardiff, with shadow Welsh secretary Christina Rees and shadow devolution minister Jim McMahon taking part alongside Andy Burnham, Steve Rotheram and Sion Simon, Labour's mayoral candidates for Greater Manchester, the Liverpool City region and the West Midlands, London deputy mayor Jules Pipe and Newcastle Council leader Nick Forbes.
The taskforce will form the basis for Labour's plans to set up a constitutional convention to look at how to take forward its proposals for a federal UK.
It will meet one of the six tests the party wants to see achieved before the UK leaves the European Union, with Labour insisting Brexit must "deliver for all nations and regions of the UK".
A joint statement from taskforce members said: "On the day the United Kingdom formally begins the process of leaving the European Union, it is clear that our constitution is no longer fit for purpose.
"The Brexit vote was in part an angry revolt of Britain's left-behind regions against the country's financial, cultural and political elites. Underlying this are deep-seated economic inequalities that drive divisions in the UK.
"The UK Government would claim for Westminster those devolved responsibilities currently administered by Brussels, increasing the concentration of power in what has until recently been one of the most centralised states in the developed world.
"As leading Labour figures from across the UK, we reject this Whitehall power grab and call on the UK Government as part of the Brexit negotiations to agree to the transfer of powers over agriculture, fisheries, regional policy and environmental protection to the Scottish Parliament and the Welsh and Northern Irish assemblies."
The group said Labour was "the party of devolution" and must seize the agenda after Brexit.
Proposals to replace the House of Lords with an elected senate of the nations and regions have already been put forward.
The statement continued: "The key to bridging social and economic inequalities lies in embracing a more decentralised United Kingdom, thus strengthening the bonds that tie our four nations together.
"It is time to continue the process that has commenced in Scotland, Wales and Northern Ireland by placing bottom-up economic power in the hands of the English regions which would boost local economies, enhance the delivery of public services and ensure that the voices of those left behind by economic growth are listened to."
Writing in the Financial Times, Mr Brown said Brexit presented an opportunity to redistribute powers - and warned a failure to do so fairly would "inflame a sense of injustice" in areas like those in the north of England which largely backed leaving the EU.
A more broadly based Council of the North, serving the 15 million residents of the North West, the North East and Yorkshire and the Humber, and composed of local authority leaders, mayors and even MPs, could be an option, he said.
Similar councils for England's other regions would co-operate with devolved administrations in Scotland, Wales, Northern Ireland and the London Assembly with a "more federal framework".
"A UK-wide constitutional convention may be the best starting point for building consensus around a much-needed redistribution of power," Mr Brown wrote.