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.

Speaking at Cardiff University, Mr Brown said it was a "very difficult day".

"We have the declaration of Brexit through the European Union," he said.

"We have a stand-off between the Scottish Government and the UK Government over the future of Scotland and a potential referendum.

"We have the potential for direct rule now in Northern Ireland.

"We have the different discontents of different regions about what is happening with Brexit and the effect of it on them.

"I've been thinking that the only things that have been moving forward this week in Britain are the clocks."

Mr Brown said he believed Brexit would force the country to face up to "vast structural inequalities" in income, wealth and power.

"We have got to start rethinking the British constitution in a way that gives more power to the nations and regions of the United Kingdom," he added.

Mr Brown called the triggering of Article 50 a "very sad day for Britain" and said he would prefer the UK to remain in the European Union.

"I think we've now got to think imaginatively about how we can make sure that some of these long-term structural inequalities in the United Kingdom are dealt with," he added.

"That means that there has got to be a big discussion on a more federal and a more decentralised constitution for the country."

Mr Brown said the UK needs to co-operate on a wide range of issues, from tax havens to the environment and pandemics.

"The challenge, it seems to me, is getting the right balance between autonomy and co-operation," he added.

"Not opting for complete autonomy or saying that we've got to have a totally federal Europe."

Ms Dugdale said the UK was under a "great deal of threat", more so than before the last independence referendum in Scotland.

"Independence is not the answer for Scotland," she said.

"It's not the union of our four nations which is inherently unfair or unjust. It's the actions of the powerful within it.

"This is a unique opportunity to revisit those arguments, to break them down and make a new argument for a reformed United Kingdom."

She added that a deal between the four nations of the UK is "equally important" to the deal struck with Europe.

"Labour must really grasp this agenda and run with it," she said.

"Nicola Sturgeon has really stopped governing and exists now only to campaign.

"The Scottish Parliament hasn't passed a law in the past 12 months - it's been in power for 10 years now.

"It's in her interests to suggest that the Scottish Parliament is now impotent to act.

"We must remind her that she has substantial powers now to use in the interests of the Scottish people."

She warned against the centralisation of power in Scotland, describing it the result of nationalism.

Mr Jones said the UK's "own internal mechanisms" would have to change following the triggering of Article 50.

"My greatest fear is that Brexit will lead to a rise in English nationalism," he said.

"That I think is bad for us all. I have no problem expressing my identity, I'm proud of being Welsh - I am Welsh above all else in reality.

"But I have no conflict in my British identity in that regard and I don't think people should have to choose like that."