It said Britain government's position has changed from being an "enthusiastic backer" of the international Arms Trade Treaty to "one of the most significant violators".

Not just Oxfam, other international humanitarian groups too have come down heavily on the UK's arms support to the Middle East. It has been repeatedly asked to stop weapons sales following reports of severe breach of humanitarian law in Yemen's bloody war.

The agency will speak at the second conference of state parties to the treaty in Geneva on Tuesday (23 August). Penny Lawrence, deputy chief executive of Oxfam, is expected to say, "UK arms and military support are fueling a brutal war in Yemen, harming the very people the Arms Trade Treaty is designed to protect. Schools, hospitals and homes have been bombed in contravention of the rules of war."

She will add, "The UK government is in denial and disarray over its arms sales to the Saudi-led coalition bombing campaign in Yemen. It has and its international credibility is in jeopardy as it commits to action on paper but does the opposite in reality."

Countries that are signatories to the Arms Trade Treaty, like the UK, are supposed to review their international arms trade to ensure the weapons they supply are not used or diverted for illegal purposes, war crimes or against human rights. It sets certain international standards for governments to adhere to.

The British government has made arms sales to Saudi worth more than £3bn for a year since March 2015, when the war in Yemen was at its peak, Oxfam said. The US had approved $1.15bn (£879m) worth weapons, while France had given arms valued at close to £14b.

The 17-month-old war, which has been dubbed as the most underreported conflict, has reportedly left more three million people displaced and over 14 million people – almost half the population of Yemen – suffering from hunger and malnutrition. The violence has also killed over 6,000 people, according to the UN.