The court will decide whether the British government failed to suspend weapons sales to the Saudi kingdom, which is fighting a war in Yemen.
The UN claims Saudi air strikes against Houthi in that country have caused thousands of civilian deaths.
The Campaign Against the Arms Trade, which is bringing the case, claims the UK has contravened humanitarian law.
Equipment sold to Saudi Arabia includes Typhoon and Tornado fighter jets, as well as precision-guided bombs.
The sales contribute to thousands of engineering jobs in the UK, and have provided billions of pounds of revenue for the British arms trade.
"Monday's verdict could have far-reaching consequences for Britain's relations with its Gulf Arab allies."
Restrictions on food and fuel imports have pushed Yemen to the brink of famine, and two million people in the country have been internally displaced.