A new rugby stadium and retail park on the outskirts of Llanelli could hit the town centre, according to a study.
Stradey Park has been home to the Scarlets since 1879
An application for a 15,000 capacity arena together with shops, restaurants, a hotel and food store was granted by Carmarthenshire councillors on Tuesday.
But retail experts have said it may affect the "vitality and viability" of town centre businesses.
The Llanelli Scarlets said the club's future as a regional side could not be sustained without the stadium move.
The club and Carmarthenshire council are behind the plans to relocate the team from Stradey Park to the new ground at Pemberton.
The retail part of the application is needed to pay for the stadium, athletics track and other sports facilities in the £45m development.
But a retail study accompanying the application to Carmarthenshire council's development control board stated: "It is inevitable that the retail elements will result in a diversion of some trade from the existing town centre."
It added the scheme "may be harmful to the town's vitality and viability."
Paul Oram, of Llanelli Chamber of Commerce, said while his organisation welcomed the new stadium it had concerns over the out-of-town retail development.
"We support the move to the new stadium because the Scarlets are a critical brand and a critical shop window for Llanelli throughout the world," he said.
"But the fact that there is going to be a big retail facility there is obviously going to have a major impact on the town centre.
"The chamber's view is yes, we support the Scarlets, but we must have a strategy for the town centre alongside the development of the Pemberton site."
The club and council staged an exhibition outlining the plans at the town hall in September.
Fifteen people living near the proposed site had objected to the application on grounds including increased traffic, loss of wildlife habitat and noise and air pollution.
Scarlets' chief executive Stuart Gallacher said before Tuesday's decision Stradey Park was in poor condition and did not have facilities to generate income on non-match days.
"Llanelli Scarlets cannot continue to maintain its history and tradition as a premier league side in the modern ear unless it moves to the new stadium," he said.
"The club would continue without the stadium but would at best become a non-premier league side."