The move would see New Look's Dorset workforce cut by half.
Clothing chain New Look is to transfer up to 250 jobs from its Dorset HQ to London as part of expansion plans.
The firm is to relocate a number of its operational activities, including the buying, merchandise and design teams, to a new site in the capital.
It is now seeking a bigger office in London to accommodate staff.
New Look has said it needs to move some operations to the capital because its competitors are based there, "making the talent pool both deeper and wider".
The move would see the firm's Dorset workforce cut by half.
Jim Knight, South Dorset MP, said: "I am disappointed by the decision of New Look to relocate 250 high-quality jobs from Weymouth to London.
"I was assured by New Look that the company will be working with staff and that those affected will be offered a relocation package."
Ease of travel
In a statement, New Look said: "Many professionals in the areas of buying, merchandise and design were unable to commit to roles with New Look that were based at the Weymouth office.
"Competition in these skilled areas is always fierce, with the precedent being amongst the majority of our peers for head office facilities to be based in either London or other European capitals.
"Ease of travel to our international businesses and partners also make a compelling case in deciding to relocate a number of divisions from Weymouth to London.
"New Look remains committed to retaining its heritage in Weymouth and plans to continue to operate its support functions from there, including most of the IT, finance, human resources, help desk and property departments."
The firm said the jobs would be moved to London over the course of this year.
In 2005, New Look closed its distribution centre in Weymouth, laying off 500 workers, but kept its head office staff in the town.