Britain is at risk of being prosecuted for war crimes because of growing evidence that missiles sold to Saudi Arabia have been used against civilian targets in Yemen’s brutal civil war, Foreign Office lawyers and diplomats have warned.

Advisers to Philip Hammond, the Foreign Secretary, have stepped up legal warnings that the sale of specialist missiles to the Saudis, deployed throughout nine months of almost daily bombing raids in west Yemen against Houthi rebels, may breach international humanitarian law.

Since March this year, bombing raids and a blockade of ports imposed by the Saudi-led coalition of Sunni Gulf states have crippled much of Yemen. Although the political aim is to dislodge Houthi Shia rebels and restore the exiled President, Abed-Rabbo Mansour Hadi, thousands of Yemeni civilians have been killed, with schools, hospitals and non-military infrastructure hit. Fuel and food shortages, according to the United Nations, have brought near famine to many parts of the country.

Amnesty International, Human Rights Watch (HRW) and other NGOs, claim there is no doubt that weapons supplied by the UK and the United States have hit Yemeni civilian targets. One senior Foreign and Commonwealth Office (FCO) legal adviser told The Independent: “The Foreign Secretary has acknowledged that some weapons supplied by the UK have been used by the Saudis in Yemen. Are our reassurances correct – that such sales are within international arms treaty rules? The answer is, sadly, not at all clear.”

The aftermath of an air strike by the Saudi-led coalition on Sanaa on 13 July; 1.5 million people have been displaced (Getty)

Although the Department for International Development recently received assurances from the Saudi government that it did not want a famine to develop on its doorstep, there is concern within the FCO that the Saudi military’s attitude to humanitarian law is careless. Officials fear that the combination of British arms sales and technical expertise used to assist bombing raids on Yemen could result in the UK being hauled before the International Criminal Court on charges relating to direct attacks on civilians.

Another government lawyer warned: “With Britain now expected to join the United States and France in the war on Isis in Syria, there will be renewed interest in the legality of the assault in Yemen. It may not be enough for the Foreign Secretary to simply restate that we have yet to carry out any detailed evaluation [of UK arms used in the bombing of Yemen].”

The legal adviser said: “Yemen could be described as a forgotten conflict. Inside the Foreign Office a course-correction is seen as crucial. It is a proxy war, with the Saudis believing Iran is behind the Houthi rebellion.”

Oliver Sprague, Amnesty International’s arms trade director, told The Independent: “There is a blatant rewriting of the rules inside the FCO. We are not supposed to supply weapons if there is a risk they could be used to violate humanitarian laws and the international arms trade treaty – which we championed. It is illogical for Philip Hammond to say there is no evidence of weapons supplied by the UK being misused, so we’ll keep selling them to the point where we learn they are being used.”

Most of Saudi’s weapons are supplied by the United States. With help from the UK, the US is also offering logistical support, airborne refuelling, with a specialist Pentagon-approved team providing intelligence on targeting. This month the Obama administration authorised a $1.29bn (£858m) Saudi request to replenish stocks of specialist missiles, a move seen by critics as an effort to assuage Saudi anger over the US-brokered nuclear deal with Iran, the kingdom’s key regional rival.

In July, Britain authorised the transfer of Paveway IV missiles from the RAF to Saudi Arabia. The MoD approved a switch in positions on an order book from the arms manufacturer, Raytheon UK.

The contract, worth close to £200m, secured the supply of hundreds of the air-launched missiles to the Saudi air force over the next two years. The Raytheon precision weapons are used by both the RAF and its Saudi counterparts on Typhoon and Tornado fighter jets, supplied by BAE Systems. The order switch ensured that the Saudi arsenal, depleted through multiple daily bombing raids on Yemen over the past nine months, would not be exhausted.

This week both Amnesty and Human Rights Watch issued new evidence, based on their own field research, which they said showed that a factory in the Sanaa governate that was not involved in any military production, was destroyed by a UK-made cruise missile.

David Mepham, the UK director of HRW, said a GM-500 air-launched missile made by the UK firm Marconi had destroyed the factory and left a civilian worker dead. He said this was only the latest “multiple well-documented case of violations of the laws of war by the Gulf coalition in Yemen. UK ministers have consistently refused to acknowledge this”.

Doubts within the FCO over the legality of the British contribution to the Saudi war in Yemen have echoes of the debate in the run-up to the Iraq war. In 2003 Elizabeth Wilmshurst, an FCO deputy legal adviser, resigned after questioning the legality of joining in the invasion of Iraq without a defined resolution from the UN.

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A Government spokeswoman said:

"We do not recognise those comments but HMG takes its arms export responsibilities very seriously and operates one of the most robust arms export control regimes in the world. We rigorously examine every application on a case-by-case basis against the Consolidated EU and National Arms Export Licensing Criteria. Risks around human rights abuses are a key part of our assessment.

"The MoD monitors alleged International Humanitarian Law (IHL) violations, using available information, which in turn informs our overall assessment of IHL compliance in Yemen. We regularly raise our concerns with the Saudis, and have repeatedly received assurances of compliance with IHL. It is important that transparent investigations are conducted into all incidents where it is alleged that IHL has been breached, and we are offering advice and training to the Saudis to demonstrate best practice and to help ensure continued compliance with International Humanitarian Law."

Asked by The Independent whether the UK government regarded relations with the Saudis as too important to risk by asking awkward questions about the bombing of Yemeni civilian targets, another FCO adviser responded: “There are many Elizabeth Wilmshursts around here at the moment. Not all are being listened to.”

The full extent of suffering inflicted on Yemen’s population by the war has been laid bare by a series of independent assessments. The aid charity Médecins Sans Frontières describes Yemen as a “country under siege” in a new report. The UN’s next humanitarian assessment of Yemen is expected to state that close to 5,000 civilians have been killed and almost 25,000 wounded since the beginning of the bombing campaign against the Houthi rebels.

The UN estimates that 21 million people now lack basic, life-sustaining services, and more than 1.5 million of them have been displaced from their homes. Unicef estimates that as many as 10 children a day are being killed, with six million people facing food insecurity. The World Food Programme says most Yemeni provinces are now classified as only one level below a full famine crisis.

Frances Guy, a former British ambassador to Yemen, described the famine and the humanitarian situation as “tragic”. She added: “We should also be talking about Yemen in the context of security, asking where is the next place that Isis will go? The answer is Yemen. Because of the instability caused by the bombing, we have helped created the next space for Isis after Syria. This is where they will retire to.”