MPs have called on businesses, residents and councils to lobby government ministers to remind them of the importance of widening the A11.
MPs believe the project is at risk in the Comprehensive Spending Review.
South West Norfolk MP Elizabeth Truss chaired an MPs' meeting and said the public inquiry report was still with the Department for Transport.
"We need to ensure it is foremost in the minds of ministers making decisions," she said.
MPs believed that the widening of a nine-mile section of road from Barton Mills in Suffolk to Thetford was to go ahead after a public inquiry.
However, the report was sent to the Department for Transport but has never been signed off so could be among the cuts.
Coalition MPs George Freeman (Mid-Norfolk), Keith Simpson (Broadland) and Simon Wright (Norwich South) attended an open meeting at Theford chaired by Ms Truss.
She told them: "There are huge growth opportunities, particularly in the Norwich-Cambridge corridor but these are being held back by inadequate infrastructure.
"I know of multinational businesses located in Thetford who are struggling to justify their presence in the town because of the cost of transporting staff and supplies.
"Dualling the A11 would change all of this.
"The projected cost is £134m. The time saved alone would be worth £1.6bn not to mention the additional businesses and jobs that would relocate.
"Despite myths that the project had been signed off and subsequently delayed it never got to that stage."
She said it was vital for businesses, residents and councils to show the government what huge growth opportunities there were should the road be widened.
Mike Brown from the campaign group Gateway A11 East said if the scheme fell victim to the cuts it would be years before the road was completed.