UPDATE: The HGV has now been recovered and all lanes are now open. There are, however, still four miles of congestion and lengthy delays are still expected as traffic clears.
National Highways says a broken down lorry forced the closure of two lanes northbound, leading to traffic crawling at an average 5mph between Milton Keynes and Northampton.
A spokesman confirmed that one of the blocked lanes had reopened at around 3.30pm but congestion is set to continue for a couple more hours at least.
Traffic is also heavy on the A5 northbound as drivers bid to beat the queues.