At 1pm today, police received a report of a motorcyclist having fallen off his yellow and purple Suzuki motorbike on the M4 westbound close to the bridge at Junction 7 for Slough West.
He has been airlifted to the John Radcliffe Hospital with life-threatening injuries.
The Highways Agency estimates delays of around two hours as a result of the accident with two lanes still closed.
Traffic is backed up to Junction 4 for the Heathrow Spur and the Agency anticipates delays until at least 6pm.
The eastbound carriageway was briefly closed while the air ambulance landed, but has since reopened.
Sgt Carl Kneale, from the Joint Operations Unit for Roads Policing, said: “I am appealing for anyone who saw the rider fall off his motorcycle.
“I would also like to hear from anyone who saw the vehicle travelling prior to the collision.”
Anyone with any information should call investigating officer PC Kevin Rodney one 101 quoting the unique reference number 610 4/4 or Crimestoppers on 0800 555 111.