Traders in County Durham claim work to transform their town centre and attract shoppers will disrupt business.
From Sunday, the way traffic moves around Darlington will be changing.
New bus stops and routes will also be put into place as well as changes to delivery areas as part of the £6.5m pedestrianisation scheme.
But businesses are accusing Darlington Borough Council of ignoring their concerns over changing delivery areas and off-street parking.
Robin Blair, vice chairman of the Darlington Market Stall Holders Association, said the effect on business would be "drastic."
He said: "People are worried about whether they will be able to deliver products to shops, and businesses will suffer.
"For the whole of the life of Darlington market, the last 143 years, there has been public transport just outside dropping off and picking up shoppers.
"This is all set to change and it is a worry. The market has always relied on spin-off trade, people waiting for a bus often pop in and even a 1% drop in trade would drastically affect the market."
In recent years Darlington has struggled to compete with bigger shopping centres in the region and council officials want to reverse this trend when the project is completed in 2008.
Richard Atlee, spokesman for the council, said: "There are big changes to bus routes and where blue badge holders can park.
"It will take a while, with a new design, but we will soon have more pedestrian space and traffic flow will be easier."