The Thelwall Viaduct near Lymm has been completely shut southbound as emergency services work to put out the blaze.
Witnesses say traffic is back up for miles because of the fire between J21 at Woolston and J20 the Lymm Interchange.Police are urging motorists to avoid the area and find alternative routes if possible warning of long delays.
Anyone stuck in the congestion is asked to stay in their vehicles while the incident is ongoing.
NW Motorway Police tweeted:
Police have now confirmed that one lane has been opened to relieve traffic.