Forty academics, among them a Nobel laureate and the co-author of one of the most cited studies on inequality's cost to health, have asked Andy Burnham to lend his government's name to a new international body tracking wealth disparities before he arrives at the UN general assembly next month. The letter leans heavily on the language he has used since becoming prime minister, invoking his own domestic fairness test as evidence that he is the right leader to champion the idea abroad. It is a shrewd piece of lobbying. It is also, whether the signatories intended it or not, a pointed reminder of unfinished business closer to home.

Burnham spent nine years as mayor of Greater Manchester building his reputation on exactly this territory. The city-region became the country's first Marmot city, commissioning Michael Marmot's Institute of Health Equity to map how housing, employment and poverty were shortening lives, and turning the findings into Making Manchester Fairer, a five-year plan aimed at social determinants rather than hospital throughput. Officials in Whitehall took notice. Earlier this year the health secretary described Burnham and his South Yorkshire counterpart as having more practical influence over what determines health in their regions than the department itself, and handed both areas a devolution trial explicitly designed to close what was called the NHS postcode lottery. That trial is still a pilot. It has not yet been extended, funded or proved at national scale.

This matters because the gap it is meant to close has not narrowed in any way that would let a prime minister claim the domestic argument is settled. Recent mapping of urban health found a nineteen-year difference in life expectancy for women living within a twenty-minute walk of each other in a single London borough. Air pollution linked to deprivation is estimated to contribute to around thirty thousand deaths a year. Damp and mould, a marker of poor housing and a driver of respiratory illness, remain several times more common in the most deprived neighbourhoods than elsewhere. Greater Manchester's own experience shows these gaps can be narrowed with sustained local effort, since it has bucked the national trend on life expectancy under Burnham's tenure. What it does not yet show is that the model travels, or that a government under fiscal pressure will fund its national rollout at the pace originally promised.

That unresolved question is what gives the inequality letter its edge. Asking Burnham to help build an international evidence body on wealth disparities, chaired by Joseph Stiglitz and modelled on the climate panel that reshaped global environmental diplomacy, is asking him to stake reputational capital on a subject where his authority currently rests on a single city-region's pilot rather than a national settlement. Health leaders inside the NHS will recognise the pattern. Neighbourhood Health, prevention funding and the shift of resource toward primary and community care have been government priorities in name for years without matching investment following at the pace the rhetoric implied. If Making Manchester Fairer remains a case study rather than the template for a national health inequalities strategy, the prime minister's global advocacy will invite exactly the scrutiny the letter's authors hope to avoid, an accusation of speaking fluently about fairness abroad while the postcode lottery persists at home.

There is also a narrower, more immediate test. Aid cuts under the previous government reduced bilateral support to some African countries by as much as ninety per cent, weakening exactly the kind of international health and development architecture an inequality panel would need to draw on for credible data and delivery partners. Reversing that trajectory, or at minimum halting further erosion, would do more to establish the seriousness of any UK commitment to the IPI than the signature itself.

None of this means Burnham should decline to back the panel. It means the letter is best read by NHS leaders and by the new health secretary as leverage aimed inward as much as outward, a challenge to convert a mayoral legacy into a funded national programme before its architect starts exporting the idea. Global credibility on inequality will be earned in Trafford and Tameside before it is earned in New York.