The scheme involves upgrading the route between Sparkford and Ilchester to relieve the over-congested road and create 30 miles of dual carriageway between Mere and South Petherton.
With over 26,000 vehicles using the route each day, the two schemes are part of the Government's £15 billion roads investment between 2015 – 2020.
Work will include making junctions safer as well as providing better links for pedestrians and cyclists.
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The consultation runs from 15 February to 29 March. The preferred route between Sparkford and Ilchester is due to be announced in autumn this year. Work is expected to start in 2020.
Transport Secretary Chris Grayling said: "Almost 4 million trips are made on the A303 every year and we are investing more than £2 billion to improve these motorists' journeys, making them quicker and safer.
"Tackling congestion on this vital road will also help boost the economy in the south-west, unlock thousands of jobs and homes and promote tourism."
Highways England chief executive Jim O'Sullivan added: "This scheme will be really good news for people travelling to and from the south-west, particularly during the busy summer months.
"Our consultation and public exhibitions will provide an excellent opportunity to explain further our plans and to hear feedback from people on our proposals to deliver the scheme."
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