A woman has been airlifted to hospital with life-threatening injuries after a knife attack near a supermarket in west London.

Three other women were stabbed during the attack in the car park of a branch of Sainsbury's in Hampton.

A man in his 60s was arrested at the scene on suspicion of attempted murder, police said.

Police and paramedics were called to the store at 10.35am and when officers arrived they found three women with knife wounds, which were not serious.

A fourth woman found at a property nearby was taken to hospital by air ambulance with life-threatening injuries, a police spokesman said. 

Police said a taser was deployed during the suspect's arrest but it was not discharged.

A woman working at a nearby pub said the attack appeared to be unprovoked.

"He only went for the women," she said.

"The man ran through and started attacking them. It didn't seem like they knew him; it was completely unprovoked.

"If it had been 10 or 20 minutes earlier there would have been children there because it's right outside a primary school. He was caught very quickly just outside the car park".