The incident happened on the No.1, Longridge to Preston service at around 09:55 on Tuesday, December 10.
Police believe that the man boarded the service at The Old Oak bus-stop. We believe he fell in the stairway, suffering a head injury.
The 70-year-old man from Longridge was taken to the Royal Preston Hospital. He died of his injuries on Wednesday.
Sgt Marc Glass of Lancashire’s Tactical Operations Division, said: “A man has died and our thoughts are with him and his family at this incredibly difficult time.“An investigation is underway to establish exactly what happened on the bus.“A Family Liaison Officer has been appointed to support the man’s family.“We are appealing to anyone who witnessed the incident and has not yet spoken with police to come forward.Anyone with information can contact police on 101 quoting log reference 0461 of December 10th.