Britain's credibility is in jeopardy over its sale of arms to Saudi Arabia, which are potentially being used in its bombing campaign in Yemen, Oxfam said on Tuesday.
The aid organization accused the British government of switching "from being an enthusiastic backer of the International Arms Trade Treaty into one of the most significant violators."
A Saudi-led coalition has been bombing rebels in Yemen since March 2015 in support of internationally recognized President Abd Rabu Mansour Hadi.
Oxfam was to use the second conference of state parties to the 2014 arms treaty on Tuesday to accuse the British government of being "in denial and disarray" over its Saudi arms sales.
"UK arms and military support are fuelling a brutal war in Yemen, harming the very people the Arms Trade Treaty is designed to protect," Penny Lawrence, Oxfam GB deputy chief executive was to tell the conference in Geneva.
"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," she was to say.
The 2014 treaty, of which Britain is a signatory, attempts to regulate the arms trade.
Earlier this year the government said it was confident that Saudi Arabia's actions in Yemen did meet the standards set by the treaty.
However it later withdrew the statement, saying it could not verify such a claim but that it had not been a deliberate attempt to mislead parliamentarians.