There are severe delays between junction 14 near Milton Keynes and junction 15 for Northampton.
Highways England has confirmed the closure follows a collision involving an HGV and a large diesel spillage.
A spokesman said: "Contractors are working at the scene to clear the spillage.
"Please seek an alternative route as 8 miles of queuing traffic on the approach."
The incident is also affecting surrounding routes including the A508 northbound where there is very slow traffic.