Officers investigating the serious collision near Dorchester are appealing for witnesses or anyone with dashcam footage to come forward.
The incident, involving a blue Land Rover Discovery and a Royal Mail van, happened on Westleaze near Charminster and was reported to police at 7.15am on Wednesday May 24.
The driver of the Land Rover, a local man in his 20s, was taken to hospital by air ambulance with life-threatening injuries.
The male driver of the van was taken to hospital with minor injuries.
A road closure was put in place to enable emergency services to safely attend the scene.
Police Sergeant Gareth Thomas, of the Roads Policing Team, said: “An investigation is underway and I would ask anyone who witnessed the collision, or manner of driving of either vehicle in the moments leading up to it, to please get in touch.
“Also, I would ask motorists who were travelling in the area with a dashcam fitted to their vehicle to please check for any footage to help my enquiries.
“Finally, I would like to thank the public for their patience and understanding during the road closure.”
Anyone with information is asked to contact Dorset Police at www.dorset.police.uk, via email at scit@dorset.pnn.police.uk or by calling 101, quoting occurrence number 55230079490. Alternatively, independent charity Crimestoppers can be contacted anonymously online at Crimestoppers-uk.org or by calling 08000 555 111