The man, in his 40s, was a passenger in a private transport ambulance when it collided with a lorry.
The accident happened on the M1’s southbound carriageway between junction 21 and 20 at about 11.40am on Tuesday January 28.
The casualty was rushed to Coventry’s University Hospital by ambulance with serious injuries.
He is still being treated in hospital and his condition is described as “serious but stable”.
Another man and two women who were also in the vehicle suffered shock and minor injuries.
They attended hospital for treatment and have since been allowed to go home.
The wagon driver was badly shaken up but wasn’t injured.
Leicestershire Police are appealing to anyone who saw the smash to contact them urgently.
PC Andy Richardson said: “The incident happened at a busy time on the motorway and so there will have been a number of people in the area at the time.
“We are carrying out enquiries to establish the full circumstances of the collision.
“We are asking anyone who has any information in relation to the incident to make contact.”
Appealing directly to motorists, he added: “Were you in the area of the collision at the time?
“Did you witness the incident or did you see either of the vehicles being driven beforehand?
“Do you have any dashcam footage from the area at the time?
“If you do have any information and have not yet spoken with us then please make contact.”
If you can help then call police on 101 quoting incident 20*52949.