It has now been confirmed that a van was in collision with a broken down lorry, according to officials.
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All emergency services - including the air ambulance - were called to the A45 eastbound just before the Earls Barton turn off just before 11.40am.
A spokesperson for Northants Fire and Rescue Service said: "A van was in collision with a broken down lorry.
"One man has been taken to hospital by air ambulance."
The westbound carriageway was temporarily closed for the air ambulance to land. That closure has now been lifted.
The eastbound stretch remains closed - between Great Billing and Earls Barton - and is excepted to be closed for a number of hours, with drivers being told to avoid the area.
There are severe delays on the eastbound carriageway between Brackmills and the crash scene.
PC 1604 Scott tweeted: "Currently on a road closure following an RTC on the #A45 eastbound between Great Billing and Earls Barton.
"Road will remain closed for a number of hours please avoid the area, very heavy standstill traffic."
Northamptonshire Police and Highways officers have spent the last few hours turning vehicles around caught in the tailbacks.
This was completed at around 3pm.
PC 1604 Scott tweeted: "We are now turning around the last of the vehicles, thank you everyone for your patience.T
"The road will remain closed for sometime however due to needing to he resurfaced."
A spokeswoman for East Midlands Ambulance Service added: "We received a call at 11.50am this morning to the A45.
"The caller reported an road traffic collision.
"We sent two paramedics in two ambulance cars, a crewed ambulance and the air ambulance. We transported one patient to Coventry and Walsgrave Hospital."