Today, the Saudi-led coalition launched a full-scale attack on the port of Hodeidah in Yemen. The Saudis and Emiratis had been urged not to do so by representatives of many humanitarian organisations. Staunch allies of Saudi Arabia such as Britain have warned of the devastating consequences for civilians who will inevitably face the full impact of this military onslaught.

Although Theresa May and Boris Johnson have urged restraint in their personal contacts with the Emirates and with Saudi crown prince Mohammed bin Salman, their warnings and fears have been ignored.

The problem for Britain is that we are complicit in this attack. It is part of the coalition that supports Saudi Arabia in its war in Yemen. Yemen is already blockaded by the Saudi coalition. Repeated warnings that a manmade famine is being generated have been ignored. Britain, as the “penholder” on Yemen at the UN security council, nevertheless takes a nakedly pro-Saudi approach to the conflict. Indeed, a recent presidential statement drafted by Britain had to be suppressed by other members of that same security council. Britain rightly condemns the Houthis for launching sporadic missile attacks on Riyadh, but stays silent on the nightly air attacks by the Saudi air force that kill innocent civilians in Yemen. Last year, I was in Sana’a, Yemen’s capital: on one night alone, there were six terrifying bombing raids.

The British government finds itself not on the side of innocent families who fear the fire that falls from above, but on the side of the perpetrator who has launched a huge military gamble to take Hodeidah from the Houthis. The echoes of what the Russians did in Syria over Aleppo ring out. The government rightly condemned the brutal attack on innocent lives in Aleppo. Where is Britain’s voice of sanity in the looming humanitarian catastrophe in Hodeidah?

This reckless assault to capture Yemen’s main port threatens the lives of hundreds of thousands of innocent civilians. It is time for the government to make clear it will no longer support what Saudi Arabia is doing in Yemen and call for an immediate ceasefire. It should align itself absolutely alongside the United Nations special envoy to Yemen, Martin Griffiths, a British international civil servant of huge experience. Any hope he has of achieving a ceasefire and the start of political negotiations is destroyed by today’s onslaught.

Wheat given by Unicef being unloaded at Hodeidah, where 70% of Yemen’s imports arrive. Photograph: Abdo Hyder/AFP/Getty Images

Indeed, cynics are saying that the whole reason for the timing of this attack on Hodeidah is to destroy any chance of Griffiths and the United Nations securing the ceasefire upon which those political talks must depend.

Be in no doubt, the immediate risk to civilians of this attack on the densely populated port is terrifying. The UN humanitarian coordinator, Lise Grande, states that “as many as 250,000 people may lose everything – even their lives”; the foreign secretary has acknowledged that 350,000 people could be forced from their homes into the desert beyond, where there is neither water nor food. The wider humanitarian risk is eye-watering: enormous numbers of innocent Yemenis will be at risk of entrapment, displacement, disease and starvation. Turning Hodeidah into a conflict zone would restrict access to the port through which a massive 70% of Yemen’s imports flow. Any disruption to vital imports will be measured in Yemeni lives. Indeed, it is doubtful whether this attack could comply with international humanitarian law.

Finally, apart from risking humanitarian catastrophe and derailing any prospect of peace, respected military experts make clear that the plan for the attack is “lunacy” – destined to fail or draw the coalition into a drawn-out and bloody battle. Even the general leading the UAE’s forces has reportedly admitted that pacifying Hodeidah, if indeed it is possible, will take a very long time. The attention of British ministers and MPs is inevitably focused elsewhere at this time, but we should be unequivocal in our message to the UAE and Saudi Arabia: they will be held accountable for any violations against civilians and breaches of the rules of war carried out by forces that they train, pay or give orders to.

Britain, along with our allies in the US and France, has unique influence to steer Saudi Arabia and the UAE away from this recklessness. Just consider what happens if Iran decides that the international rule-based system is not standing up for international humanitarian law and intervenes further. As supporters of the Saudi/UAE–led coalition and key arms suppliers, we bear a unique responsibility. We cannot look the other way as this catastrophe in Yemen unfolds. We must stand true to our values, to strategic common sense, indeed true to our allies’ best interests and make clear that we can no longer support their war in Yemen.

Thousands of Yemeni lives may hang on us doing the right thing. All our energies in this matter must be dedicated to supporting the United Nations in achieving a ceasefire and the start of the political negotiations upon which the future of this poor, tragic and beautiful country depend.

• Andrew Mitchell is Conservative MP for Sutton Coldfield