Costed at around £700m, the project will be the biggest smart motorway upgrade yet and involve replacing 16 bridges.
The Highways Agency has just formally started public consultation in order to apply for development consent to the Planning Inspectorate in early 2015.
Subject to receiving development consent from the Transport Secretary construction work is expected to start in September 2016 and take over five years to complete.
The smart motorway scheme from junctions 3 to 12 will include the permanent conversion of the hard shoulder to a running lane.
The carriageway between Heathrow junction 4 and junction 4b with the M25 is currently four lanes wide, with a hard shoulder and will be widened to five lanes.
