Of all the numerous and diverse resolutions that the UN security council has debated over the last six months, you would think that few could have been as pressing as securing a ceasefire in Yemen.

Indeed the UK, which officially “holds the pen” on the Yemen issue, circulated a draft resolution proposing a ceasefire and a roadmap for lasting peace back in October.

However, this was the subject of an immediate veto and has not seen the light of day since, despite the UK chairing a security council meeting on Yemen just last week.

The draft was vetoed not by Russia or any other members of the security council with the actual power to do so, but by Saudi Arabia, whose UN ambassador explained: “There is a continuous and joint agreement with Britain concerning the draft resolution and whether there is a need for it or not.”

Six months on, the need for peace is more urgent than ever, so you would have hoped the prime minister would have used her trip to the Gulf this week to tell the Saudis that – whatever objections they may have – Britain must sever its “continuous and joint agreement” and present the resolution immediately.

No such luck. Instead, the prime minister defiantly set out the so-called “May doctrine” for foreign policy. The upshot was that “our British national interest” is really the only thing that matters, and no amount of “sniping” about the catastrophe in Yemen should be allowed to get in the way of our trading relationship with the Saudis. In other words, there is no place in Theresa May’s doctrine for old-fashioned considerations like human rights and humanitarian crises.

But if we can’t rely on Tory ministers to respect such values, we can at least still look to the law. Thanks to the leading role played by Britain under a Labour government, it is now a key principle that arms sales should not be permitted if there is a “clear risk” of their being used to violate humanitarian law.

Asking whether that is the case with the £3bn worth of weapons Britain has sold to Saudi Arabia since the start of the Yemen conflict is not, therefore, sniping; it is better described as our moral and legal duty.

What began in 2015 as a legitimate, UN-backed effort by a Saudi-led coalition to overturn the Houthi rebellion and restore Yemen’s elected government, quickly – in foreign secretary Boris Johnson’s own words – turned into a proxy war to extend Saudi control in the region. Thousands of civilians have been killed, a large number at the hands of the Houthis, but the majority due to coalition air strikes. There has been no safety for Yemenis at weddings and funerals, in hospitals and schools, or even in their own homes.

At a time when Yemen faces starvation, the Saudi-led forces have also destroyed cattle farms, food factories, water wells and marketplaces, along with the ports, airports, roads and bridges on which effective supplies of food aid depend.

One of the fundamental pillars of international humanitarian law is that proper distinction should be made between military targets and civilians. That is why indiscriminate bombing, let alone the deliberate targeting of residential areas or agricultural infrastructure, is considered a war crime.

Have such crimes been committed by the Saudi coalition in Yemen? We cannot know for sure, and we will not know until there is a full, independent, UN-led investigation into all the thousands of airstrikes on civilian areas and each of the deaths they have caused.

You would think the UK government would be leading the calls for such an investigation, but no again; instead, and typically, it has been content to say that the Saudis should be left to investigate themselves, and whatever they conclude will be accepted as gospel.

No doubt that was the extent of May’s comments on the subject today, before she got down to the only issue that really matters to her: trade.

That may be her doctrine, but it is not the law, and the British courts will soon rule on whether her government has been right to judge that there is “no clear risk” that our weapons have been used in breach of international law.

It will be shameful if that is what it takes to get them to do the decent thing on Yemen. These are: to halt further arms sales to Saudi Arabia, pending a full and genuinely impartial investigation into alleged war crimes; to ignore the Saudi veto and present a resolution for a ceasefire and lasting roadmap for peace at the UN without further delay; and to give the impoverished people of Yemen the peace they so desperately need.

They may have nothing to offer May in terms of trade. But our foreign policy should always be about more than that.