The High Court rules on Monday whether the Government has unlawfully failed to suspend the sale of UK arms to Saudi Arabia.
The case is being brought by the Campaign Against Arms Trade (CAAT), which says UK fighter jets and bombs sent to the Gulf state have been used in the conflict in Yemen in which thousands have died.
The group is attacking the Government's refusal to suspend export licences for the sale or transfer of arms and military equipment to Saudi Arabia and decisions to continue granting new licences.
The case is being seen as potentially having a wide impact on the multibillion-pound arms trade generally, which could become even more important to the economy in post-Brexit Britain.
CAAT said more than 10,000 people have been killed as a Saudi Arabia-led coalition intervenes in the Yemeni civil war and is guilty of "repeated and serious breaches" of international humanitarian law.
It said the fighting has created a humanitarian catastrophe, destroying vital infrastructure and leaving 80% of the population in need of aid.
But the UK has continued to allow sales, with over £3.3 billion worth of arms having been licensed since the bombing began in March 2015.
The Government is arguing there is no "clear risk" that UK licensed items might be used to commit a serious violation of humanitarian law.
Government lawyers say in a written statement before the court that it is relying on "all of the information available" - some of which "may not be publicly available".
Lord Justice Burnett and Mr Justice Haddon-Cave were told the Secretary of State for International Trade, who is defending the Government stance, is "relying considerably on sensitive material" the disclosure of which in open court "would be damaging to national security".
But Martin Chamberlain QC, appearing for CAAT, said the evidence to be presented in open court was enough on its own to show that "no reasonable decision maker" could have allowed the sale of arms to Saudi Arabia to continue.
Mr Chamberlain said the evidence included a large number of authoritative reports and findings from bodies including Amnesty International, Human Rights Watch, Oxfam and Rights Watch (UK) who were making submissions to the court.
CAAT was also relying on reports from an expert panel appointed under a UN Security Council resolution, other UN bodies and officials and from the EU Parliament.
The QC said the findings of those bodies "establish an overwhelming case that the Kingdom of Saudi Arabia-led (KSA) coalition has committed repeated and serious breaches of international humanitarian law in Yemen".