The country's second largest supermarket chain previously used the City agency to support its financial and corporate affairs in the UK.
Asda confirmed in a statement that it had 'stopped working with the Brunswick London office' and taken the agency's work in-house.
The statement said the plan to centralise financial comms had been worked towards alongside Brunswick for a number of months and that the relationship with Brunswick had ended at the beginning of March.
Asda, part of the Walmart empire, added that it had 'no plans' to replace Brunswick on the account.
It is understood that Brunswick will continue to work with Walmart in South Africa to support comms around Walmart's $2.3bn takeover of Massmart.
A hearing by South Africa's competition watchdog into the acquisition was postponed last week, after the South African government and unions asked for more time to submit additional information.
It is also understood that Brunswick will continue to hold the retained financial PR account for Asda Retail Development, the company's property and real estate investment arm.
Current CEO Andy Clarke took over from predecessor Andy Bond in May last year.
Clarke took charge as Asda was losing ground to its rivals, temporarily seeing Sainsbury's edge ahead to become the UK's second largest supermarket in December, according to industry data from Kantar Worldpanel.
Asda recently reported like-for-like sales in the final quarter of 2010 were up 2.6 per cent, its best quarterly performance of the year and ahead of all its rivals bar Sainsbury's.
Freud Communications holds Asda's retained consumer PR account, which is thought to remain unaffected by the news.