Twenty years ago, Tony Blair acknowledged the British government’s responsibility for the Irish famine that killed one million people: a healing gesture needed because, even after a century and a half, pain and anger endured and the responsibility of “those who governed in London” remained glaring. Now we are on the brink of another famine – perhaps the worst for decades, says a UN aid chief – and Britain must again bear blame. The UN called Yemen the world’s worst humanitarian crisis even before Saudi Arabia decided to blockade the country a week and a half ago, shutting out food and medicine. Now the heads of three key agencies have warned that millions are on the brink of starvation. Unicef fears that 150,000 children could die by the end of the year. A cholera outbreak that has already affected 900,000 is expected to flare up again, as the lack of fuel shuts off water and sewage systems. Twenty million people, more than two-thirds of the population, are in urgent need of humanitarian supplies.

An impoverished country has been destroyed by what is both a civil and a proxy war. Houthi rebels, allied to Iran, drove out the internationally recognised president Abd-Rabbu Mansour Hadi, allying with his predecessor who had been ousted in the Arab spring. Since then, 10,000 lives have been lost, many to heavy bombing by the Saudi Arabian-led coalition, with arms and military support from the US, UK and others. The blockade has taken this terrible, futile conflict to a new depth. It seeks to starve a population into submission – a crime against humanity horrifically familiar from its ongoing use in Syria as well as elsewhere. Britain’s staunch support for Riyadh makes it complicit.

The Saudis said a complete border freeze was needed to prevent arms shipments after a Houthi-fired missile was shot down near Riyadh, though the UN inspects inbound cargo, and a panel reportedly found no evidence to support Saudi claims of transfers. Despite partial relaxation, humanitarian aid is still barred – with very minimal exceptions in some coalition-held areas – and the Saudis say the crucial western port city of Hodeidah will not reopen until it is in UN, not Houthi, hands. All this exacerbates already desperate need across both rebel- and coalition-controlled areas. There is food in markets. But the costs of getting it there in a war, the currency’s devaluation and the long-running inability to pay government wages or even minimal welfare have left Yemenis with no way of paying for it.

The ultimate solution is political, of course. But prospects for a deal look even further away than they did a year ago. Players on the ground are profiting from the war, while others starve. Saudi Arabia’s rivalry with Iran is intensifying and expanding. Mohammed bin Salman, the now crown prince who led the charge into Yemen, is ever more powerful at home and ever more bullish abroad. Donald Trump appears to have given him a free pass to act as he will. Britain is resolute in its long-term alliance with Saudi Arabia and its determination to shore up arms exports – £4.6bn worth of weapons sales to Riyadh have been licensed since the war in Yemen began – especially with Brexit pending.

Claims that western involvement has moderated Saudi behaviour look ever more threadbare. Whatever pressure governments are exerting privately, Riyadh knows that Yemen is near the bottom of their priority list for the region. Public statements have been pitiful. The Foreign Office condemned the attempted missile attack on Riyadh within hours and voiced concern about reports that Iran had provided the Houthis with missiles four days later. But it waited a full 10 days to address the blockade – and then in terms which minimised Saudi culpability. DfiD boasts of aid might be laughable were they not so insulting in the circumstances. Britain should stop selling arms to Riyadh and shout out the need to end the blockade. To do any less is wrong and shameful. History will not be kind.