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Long delays cleared after police were called to deal with 15 migrants found in the back of a lorry on the Essex side of the M25.
Traffic tailed back from J1A for Erith and Swanscombe back to the Lakeside turn-off at J30 into Essex.
On the Kent side, the tailbacks stretched to J4 for the A21 and Orpington at its height.
An Essex Police spokesman said officers went to the slip road near J31 of the M25 after being called by several motorists with reports of migrants in the back of a lorry.
They found 15 men and women in the back of a vehicle, who appeared to have been there for some time.
Paramedics have also been at the scene, and the migrants have been taken to hospitals for check-ups.
All traffic was held for around 15 minutes just before 3.30pm.