Oxfam will today accuse ministers of being in "denial and disarray" over the selling of arms to Saudi Arabia for potential use in Yemen's bloody civil war.
Fighting in the country pits the Yemeni government, backed by Saudi Arabia, against Shiite Yemeni rebels.
The UK Government has faced repeated calls to impose a ban on the sale of weapons to Saudi Arabia amid concerns that international humanitarian law (IHL) could be being broken in the conflict.
Its apparent reluctance to do so has prompted Oxfam to claim the Government has switched from being an "enthusiastic backer" of the Arms Trade Treaty to "one of the most significant violators".
The treaty, of which the UK is a signatory, seeks to regulate the international weapons trade.
The charity will use the second conference of states party to the treaty in Geneva on Tuesday to attack the Government's stance.
Penny Lawrence, deputy chief executive of Oxfam GB, will say: "UK arms and military support are fuelling 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.
"The UK government is in denial and disarray over its arms sales to the Saudi-led coalition bombing campaign in Yemen.
"It has misled its own parliament about its oversight of arms sales and its international credibility is in jeopardy as it commits to action on paper but does the opposite in reality.
"How can the Government insist that others abide by a treaty it helped set up if it flagrantly ignores it?"
Earlier this year the Government said it was confident that Saudi Arabia's intervention in the country did meet the terms of IHL.
However, it later corrected those statements and said assessments to verify such a claim had not been undertaken but insisted the original statements resulted from error and were not a deliberate attempt to mislead MPs.
Oxfam estimates that there are more than 21 million people in need of humanitarian assistance in Yemen – more people than any other country in the world.
The UN has estimated that more than 6,000 people have lost their lives in the war while millions have had to leave their homes.
Concerns have been expressed about the way in which the conflict is being fought on both sides, but the UN has estimated that the Saudi-led coalition is responsible for twice as many civilian casualties as the other forces combined.
A Government spokeswoman said: "The UK Government takes its arms export responsibilities very seriously and operates one of the most robust arms export control regimes in the world.
"The Government is satisfied that extant licences for Saudi Arabia are compliant with the UK's export licensing criteria.
"The key test for our continued arms exports to Saudi Arabia in relation to international humanitarian law (IHL) is whether there is a clear risk that those weapons might be used in a serious violation of IHL. The situation is kept under careful and continual review."