Marks and Spencer has agreed a deal to open a store at a Lancashire retail centre after previously saying it was "reconsidering" its plans for the site.
Chorley Council said the chain has now formally signed up to the development.
Alistair Bradley, leader of the council, said the news represented a "major boost" for Chorley town centre.
"We've all seen in the media that new store openings are at a premium so for Marks and Spencer to choose Chorley shows they have a lot of confidence in what we are doing," he said.
Work on the extension, which will also include a cinema, will start in October.
The council said it is in discussions with operators for the remaining six units and it expects to fill the majority by the time it opens next year.
Market Walk was purchased by Chorley Council for £23m in 2013.