Prime Minister Andy Burnham has said his sweeping programme of devolution strengthens the case for a written UK constitution, a move that would fundamentally alter the workings of a state that has developed over centuries without a single codified rulebook.

Speaking this weekend, Burnham argued that his government's programme of decentralisation – which includes giving regional leaders far greater control over their finances – bolsters the argument for a codified arrangement, in what would be a potentially historic shift for one of the few major democracies that has never adopted a written constitution.

"It will increase the case for a constitution because Britain is changing through this, England is certainly changing through this," he said.

The intervention marks the clearest signal yet that Burnham, who became Prime Minister on 20 July after Sir Keir Starmer's resignation, intends to pair his economic reforms with a broader constitutional overhaul. He said the UK needed to devise a codified model of devolution that works for Wales, Scotland, Northern Ireland and the English regions.

"In the end, I think it does start to point towards a new constitutional settlement … with that commitment to equivalent living standards," he said, adding that his pledge of "good growth in every postcode" was "more than just a soundbite".

Burnham described the further devolution measures, outlined last week, as the most radical departure from Britain's overcentralised political establishment. The "rewiring" of the UK, he said, had "gone to a new level of seriousness" and was centred on shifting decision-making outside Whitehall to mayors and local authorities through No 10 North, the Manchester office of Downing Street he established to co-ordinate the programme.

At the heart of the plan is a significant transfer of fiscal power. From 2027, councils will keep business rates raised locally, and from early 2028 regional mayors across England will be able to retain a share of the income tax generated in their area. The government has described the move as the biggest transfer of power for a generation. The reforms will not change the rate of income tax paid by residents, but rather how revenues are allocated between Westminster and local government.

Under proposals to be published in late October, it is expected that economic powerhouses such as London, Greater Manchester and the West Midlands will hand some of their share to less prosperous regions such as Hull and East Yorkshire. The full details, including the proportion of taxes local leaders will keep, are set to be released after the Budget on 28 October.

The plan applies only to England, though the government has said it will work with the devolved administrations in Scotland, Wales and Northern Ireland to achieve similar outcomes. Devolution across England remains uneven: around half of the population does not live in an area covered by a metro mayor.

The intellectual roots of Burnham's agenda lie in his 2024 book, Head North, co-authored with the Liverpool city region mayor, Steve Rotheram. In it, Burnham argued that deep inequality had become entrenched in Britain partly because the country had not evolved from a "feudal state" in which power is largely concentrated in Westminster.

"Our failure fully to set out the rules for how the country should be run is one of the principal reasons why Britain is one of the most unequal major countries in the world," he wrote. "Our unwritten and murky constitutional arrangements have had the effect of hoarding power in a small number of hands – in Whitehall and Westminster and, by extension, in the powerful networks of institutions and vested interests that surround them."

Burnham has cited as inspiration Germany's postwar Basic Law, or Grundgesetz, which enshrines the principle of "equivalent living conditions" across the country's states, with wealthier regions redistributing funds across the nation.

However, constitutional experts have cautioned that the scale of such a change should not be underestimated. Tim Bale, a professor of politics at Queen Mary University of London, said introducing a written constitution would be a "huge departure" for the UK, noting that most countries had done so "in the wake of a revolution or a war that has removed or delegitimised the former regime".

While it would bring the UK into line with democracies such as the United States, Bale said a codified constitution could "represent a major constraint on the executive … because it would presumably put the rules of the game beyond the power of parliament to change with a simple majority". He added: "One of the upsides is that it might better define the competencies of the various levels of government – local, regional and national. But there is no way he will be able to limit it to that."

Dr Hannah White, chief executive of the Institute for Government, echoed the note of caution. "Adopting a written constitution – of the kind almost every other country already has – would represent a very significant constitutional shift for the UK and be a major exercise," she said. "A fully codified constitution would need to go far beyond devolution to encompass all the rules which structure the UK state."

Opposition figures have questioned whether the reforms address voters' immediate concerns. Robert Jenrick, Reform UK's Treasury spokesman, said Burnham "talks a good talk, but there's no substance behind it at all", and criticised the Prime Minister's assessment that his mission to revive growth and living standards could take a decade. "People can't wait 10 years," he said, setting out an alternative model based on lower taxes and lower energy prices rather than devolution and targeted government intervention.

It remains unclear how the government would pursue such significant constitutional change while simultaneously advancing Burnham's wider agenda on devolution, social care and public control of utilities. A particular complication is Wales, a nation in its own right, whose constitutional status sits awkwardly within a devolution framework built primarily around English regions.

Burnham, 56, a former cabinet minister who left Westminster in 2017 to become mayor of Greater Manchester, was acclaimed Labour leader on 17 July after securing the nominations of more than 94% of the party's MPs. On becoming Prime Minister, he said his generation of politicians needed to "raise our game", acknowledging that "many voters have grown frustrated with politics".

Further detail on the fiscal architecture underpinning the plans – including how much revenue mayors will retain, which Whitehall grants will be replaced and how areas with smaller tax bases will be protected – is expected alongside the Budget on 28 October.