The BBC has learned that a truckload of smuggled surface-to-air missiles has been seized by police in Saudi Arabia.
The vehicle was intercepted by the Saudi authorities last month near the port city of Jeddah and its contents are thought to have been destined for militants linked to al-Qaeda.
The seizure has been confirmed to the BBC by both Saudi and British officials.
British Airways suspended its flights to the country in August due to a separate security threat in Riyadh.
The airline has been on the point of resuming the service.
On a desert road close to Jeddah, Saudi police made an unexpected discovery last month.
In a trailer designed for carrying cars they found a consignment of missiles each powerful enough to bring down an airliner.
Dr Saad al-Fagih of the London-based Movement for Islamic Reform said the discovery was made by chance but that Special Security forces were then called to the scene.
It was later concluded that the weapons had been smuggled in from Yemen, destined for use by extremists.
The seizure follows the discovery in August of moves by an extremist cell to put Riyadh airport under surveillance.
Documents were found in a car revealing that members had mapped out the exact location of checkpoints and other security measures.
At least 25 people were killed in a bomb attack in Riyadh in May
This coincided with the testimony given by an al-Qaeda suspect that an attack was being planned on British interests in the country.
The Saudi authorities passed the information on to the British government and on 13 August British Airways suspended its regular flights to Riyadh and Jeddah.
Since then, the airline says it has had good co-operation from the Saudi authorities, who have taken a number of security precautions.
British Airways says it is now on the verge of resuming its flights to Saudi Arabia.
Since the triple bombing of Western housing compounds in Riyadh in May, the Saudi authorities have gone to great lengths to tackle armed extremists thought to be linked to al-Qaeda.
They have set up checkpoints all over the country and arrested hundreds of suspects.
They have also removed more than a thousand anti-Western preachers from their mosques and have been training them to be conciliatory towards non-Muslims.
However, senior Saudi officials have admitted privately to the BBC that they cannot be sure they can prevent another attack on the scale of the Riyadh bombings.