Eastern Road road was shut northbound for three hours yesterday following the incident, involving a Volkswagen car and a motorcycle, opposite the Good Companion Pub just after midday.
An air ambulance landed by the road shortly after the incident and the motorcyclist, a man, was flown to Southampton General Hospital.
A spokeswoman for South Central Ambulance Service said: ‘We received a call at 12.06pm about a road traffic incident involving a car and a motorcycle.
‘We sent a Thames Valley air ambulance, an ambulance crew, helicopter rapid response vehicle, another rapid response vehicle and an ambulance officer to the scene.
‘The motorcyclist sustained life-threatening injuries in the incident and was flown to Southampton General Hospital by air ambulance.’
Stagecoach South apologised for passengers for delays to services through Copnor and Milton while emergency services cleared and subsequently investigated the scene.