The A40 in Hillingdon came to a standstill after a collision involving several cars and two motorcycles.
A man in his 40s was rushed to hospital by London Ambulance Service (LAS) with multiple injuries.
An LAS spokesperson said: "We were called at 3.47pm to reports of a road traffic collision involving several cars and two motorcycles at the A40 near the Polish War Memorial.
"We sent a number of resources to the scene including the hazardous area response team (HART) and an incident officer.
"We treated a man reported to be in his forties for multiple injuries.
"He was taken as a priority to St Mary's Hospital, Paddington, a major trauma centre."
Metropolitan Police were called to reports of the collision at around 3:45pm on Thursday, February 4.
Motorists have been stuck in standstill traffic with one stuck for an hour.
All lanes of the A40 Western Avenue were closed eastbound with traffic delays as far as Denham.
Adam Lorenzo, an eyewitness, told getwestlondon: "From what we're being told it was a bike and a car at the speed cameras just before Target roundabout.
"I've been stuck for almost an hour at Uxbridge roundabout and the traffic goes as far as the eye can see.
"All lanes are closed and police are trying to divert to B467 Ickenham ."
London Fire Brigade was also confirmed to have attended and sent one engine to free any trapped persons.
A spokesperson confirmed that the brigade were at the scene for about an hour but no one was trapped.
