CHI handled the advertising for Westfield's West London site and its Stratford City development, which is due to open in September, after winning a pitch in April 2010.
Leo Burnett had also pitched for the business and it is understood that the agency is now in contention for the business along with Westfield's PR agency, Yellow Door, which was founded by Mary Portas.
CHI declined to comment on the reasons for the split with Westfield.
A spokeswoman for the agency said: "We have been advised by our lawyers not to comment at this stage."
Before appointing CHI, Westfield used Adam & Eve to produce its advertising.
The agency secured the launch campaign for Westfield West London in March 2008, but was forced to resign the business in April last year after it picked up the £20 million John Lewis account.
In September last year, CHI, in partnership with Made by Many and The Viral Factory, was also appointed to the digital account for Westfield Stratford City, which has a catchment area of 4.1 million people and will house branches of John Lewis, Waitrose and Marks & Spencer.