The biker fell onto the carriageway near junction 9 (Clayton Brook Interchange) as he was riding a chopper motorbike at around 8.50 yesterday (August 22).
The man, aged 45 and from Clitheroe, was taken to Royal Preston Hospital with serious chest and shoulder injuries.
The M61 was closed for more than three hours while accident investigators attended the scene.
Sgt Andy Halliwell, of Lancashire Police' s Tactical Operations, said: "We are urging anyone who saw what happened to come forward.
"If you can help the police please call 101 or email [email protected] quoting log 0510 of August 22."
You can also report any information online at the Lancashire Police website by clicking HERE.
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