A HGV tipped over after moving out of the way to allow an emergency vehicle past, police have said.
Initial reports suggested two vehicles were involved but police have now confirmed it was a single vehicle incident.
Emergency services were called at 8.50am this morning (Tuesday) to the A66 near Gillhead Farm, Threlkeld.
A Cumbria Police spokeswoman said: "Initial reports suggested that two HGVs had collided, but after clarification, only one HGV was involved.
"It did not collide with another vehicle but it is believed that it tipped after manoeuvring to allow an emergency vehicle past."
The driver, aged 43, from Manchester, was taken to Carlisle Infirmary for treatment to injuries which were not life-threatening.
The HGV was a tanker which was carrying an non-hazardous substance.
A Cumbria Fire and Rescue spokesman, who sent two fire engines from Penrith and one from Keswick, said the vehicle contained non hazardous material.
"Crews assisted by providing environmental protection to the recovery company. They also made the scene safe when two cars in the tail back collided," he said.
Westbound traffic on the A66 is now moving freely now but diversions remain in place for Eastbound traffic.
Those going East should follow the A591 northwards from Kendal to Bothe, then take the A595 northeast towards Carlisle A6.
The road will be closed in both directions at approximately 6.30pm for around an hour as the tanker is removed.