A five-year-old was seriously injured after five cars collided following the diesel spillage.
The child was flown to hospital by air ambulance after the incident.
There were several other casualties taken to hospital by road ambulances.
In another incident on the northbound carriageway, shortly before 7.30pm, firefighters were called by paramedics to the scene of a collision involving a vehicle which had collided with the central reservation between junction 30 and 31.
One woman was reported trapped, however when crews arrived at the scene the casualty was already out of the vehicle.
Crews assisted Traffic Control Officers in moving the vehicle over to the hard shoulder.
The diesel spillage was the cause of the incidents, say police.
Although Highways England contractors worked through the evening and into the early hours of the morning to clean the diesel spill, between junction 29 and junction 31a of the northbound carriageway, a three kilometre section of lane 1 around and beyond junction 31 was too badly damaged to be reopened safely and will need to be resurfaced over the next two or three nights.
The work will need to be done overnight, when traffic is at its lightest, as lane 2 will also need to be closed for the work to be done safely.
The damaged section passes the point where the northbound entry slip road joins the carriageway at junction 31 so the entry slip road will remain closed until the repair can be completed - probably by tomorrow morning. At least one further overnight closure will be needed to resurface all the damaged lengths of carriageway.
Drivers in Preston needing to join the northbound M6 are advised to seek alternative junctions or join the southbound M6 at junction 31, leaving at junction 29 and using the junction roundabout to join the northbound M6 from there.
Drivers can check travel conditions before setting out on journeys. Highways England provides live traffic information via its website www.trafficengland.com, local and national radio travel bulletins, electronic road signs and mobile platforms, such as the iPhone.
Information is also available from Twitter via https://twitter.com/highwaysnwest as well as services in other regions and at http://www.highways.gov.uk/traffic-information
Providing they can do so safely, road users can also call the Highways England customer contact centre on 0300 123 5000 to keep up to date with road conditions.