An out-of-control vehicle narrowly missed cars and pedestrians before ploughing into a tree outside Milton Keynes Hospital’s A&E department.
The male driver, who is in his 70s, is thought to have taken ill at the wheel at around 2.30pm on Monday.
He remains in a life-threatening condition in hospital.
One witness said: “Our taxi driver was just coming to pick us up at the time, and told us the driver of the vehicle narrowly missed him and was slumped over the wheel when he collided with bollards and a sign before hitting the tree.
“I witnessed the paramedics giving chest compressions to the driver by the side of the road moments after the collision.”
A hospital spokesman did not want to comment further on the matter.