The M6 is expected to be closed for several hours after a woman was found dead.
Both carriageways of the motorway were closed between junctions 23 at Haydock and 26 at Orrell, near Wigan, from shortly after 08:00 GMT, causing miles of tailbacks.
Greater Manchester Police said it was called following concerns for the welfare of a woman at about 07:42 GMT but she was found dead at the scene.
National Highways North West said it could not estimate when the motorway would re-open and expected it to remain closed “throughout the day”.
The agency said there were delays of about 60 minutes on the approach to the northbound closure, with five miles of congestion.
Southbound, it said there were 30-minute delays, with four miles of jams.
GMP said: “We would encourage a review of travel plans and ask for patience as emergency services conduct their enquiries.”