The Wallasey-bound tube of the Kingsway Tunnel on Merseyside has been closed after a head-on crash.
A BMW and a Rover collided just before 0300 BST in a contraflow lane.
A woman, 19, a passenger in the back of the Rover, suffered serious head injuries and is being treated at Arrowe Park Hospital.
The BMW driver, aged 69, suffered minor injuries, while a man, 45, driving the Rover and a second female passenger, aged 20, had cuts and bruises.
Motorists have been advised by police to avoid using the Kingsway Tunnel if possible and to allow more time to complete their journeys if using the Queensway Tunnel to travel between Wirral and Liverpool.
A contraflow lane has been set up in the unaffected, Liverpool tube of the Kingsway Tunnel, but police said traffic is heavy.
Police are investigating the cause of the accident.
A Merseyside Police spokeswoman said: "The collision took place while the Wallasey to Liverpool side of the tunnel was closed for maintenance, requiring traffic to travel in both directions along contraflow lanes in the Wallasey-bound tube.
"This was not a case of a motorist driving down one of the tubes in the wrong direction." Source BBC