The public's trust in the way the UK is run is breaking down, former Labour prime minister Gordon Brown has warned.
He said Covid-19 had exposed "tensions" between Whitehall and the nations and regions, who were often treated by the centre as if they were "invisible".
Mr Brown is urging Prime Minister Boris Johnson to set up a commission to review how the country is governed.
Polls have suggested rising support for Scottish independence and potentially for a border vote in Northern Ireland.
His intervention comes amid a looming clash between Mr Johnson and Scottish First Minister Nicola Sturgeon, who has demanded the UK agree to another Scottish independence referendum if the SNP wins May's election.
The Court of Session is hearing arguments about whether Holyrood can legislate to hold one even if the UK government continues to object.
Mr Johnson has repeatedly stated his opposition to another referendum while a cabinet spokesman said the Scottish public wanted to see British politicians "working in partnership to focus on defeating coronavirus".
Writing in the Daily Telegraph, Mr Brown - who advocates a federal system with more power for nations and regions - says the pandemic has "brought to the surface tensions and grievances that have been simmering for years" between Downing Street and the various parts of the UK.
He points to "bitter disputes" over issues such as lockdown restrictions and furlough and said unless underlying tensions were resolved, the UK risked becoming a "failed state".
In an interview with BBC Radio 4's Today, he said at a time "when all should be pulling together and intensifying co-operation across the UK" there was division and claims by the leaders of Scotland and Wales and the English regions that they were not being properly consulted.
Last year there were rows between the government and local authorities over coronavirus tiers, with the Labour mayor of Greater Manchester, Andy Burnham, objecting to plans to put the region into the strictest level of restrictions.
'Fed up'
Mr Brown told Today that while he was "confident" that Scotland would still be part of the UK in ten years time, the way the UK was governed had to change.
"I think the public are fed up. I think in many ways, they feel they are being treated as second class citizens, particularly in the outlying areas, that they are invisible and forgotten."
"Something has broken down in trust and has to be repaired."
Mr Brown is advising the Labour Party on its devolution strategy - but has also held talks with government ministers including Michael Gove in recent weeks.
Government sources say they are focused on taking tangible steps to demonstrate the value of the UK.
The idea of a fundamental review of the UK's power structures has been suggested as one possible way to counter support for Scottish independence ahead of May's Holyrood election.
But a series of polls now suggest support for independence is higher than support for the union - and First Minister Nicola Sturgeon will demand another referendum if, as seems likely, her party - the SNP - wins in May.
He is calling on Boris Johnson to immediately set up a commission on democracy to review how the UK is governed, something the Conservatives promised in their manifesto before the last general election.
In his Telegraph article, he suggests it would find that the UK needs a Forum of the Nations and Regions, citizens' assemblies, and a greater focus on the benefits of cooperation in areas such as the NHS and the armed forces.
Greater Manchester mayor Andy Burnham said there was no credible plan to narrow the North-South divide.
"Without a massive transfer of resources & powers out of Westminster & into the English regions, "levelling up" will be the new "big society" - a slogan without any substance," he tweeted.
Mr Brown's assertion that the "status quo is not working" echoes recent remarks made by the current Labour leader, Sir Keir Starmer,, who supports devolving more powers from Westminster but opposes another Scottish independence referendum.
The SNP said last week that there would be a "legal referendum" after the pandemic if May's Holyrood election returned a pro-independence majority.
A cabinet spokesman said the 2014 referendum "settled" the issue and the UK government's top priority remained tackling Covid-19 and "supporting jobs and businesses across all four nations".
Wales and Northern Ireland
Meanwhile, a poll commissioned by the Sunday Times in Northern Ireland found 51% of people wanted a referendum on Irish unity in the next five years.
DUP leader Arlene Foster said such a vote would be "absolutely reckless".
Numbers supporting Wales breaking away from the UK also appear to be rising.
The pro-independence campaign group Yes Cymru has said membership swelled from 2,000 at the start of 2020 to more than 17,000.
Plaid Cymru has also promised to hold an independence referendum if it wins the next Senedd election.