Wonga, the controversial payday loans company, is reviewing its UK advertising business, which is currently handled by Albion.
The lender, which is well-known for its short-term, high-interest loans, has approached AAR to help with a review that launched this week.
Albion has decided not to repitch for the account and criticised Wonga for behaving unethically towards its customers.
In 2014 Wonga agreed with the Financial Conduct Authority to pay £2.6 million in compensation for using fake legal letters to chase customers for debts.
A spokesman for Albion said: "We’ve worked with Wonga since 2011. We created some memorable work in that time across the Wonga brand, product and communications that took them from a disruptive start-up to the mainstream.
"However, there have been certain practices that we now know went on before we worked with the business and then during the tenure of our relationship that we were unaware of and that we categorically do not agree with.
"We’ve decided it’s the right time to now cut ties and therefore not repitch for the business, but we wish Wonga well in the next stage of their journey."
Andy Haste, the former chief executive of RSA, was named Wonga’s chairman in July 2014 and vowed to clean up the business. He pulled all UK TV advertising and said Wonga would not use the Wongies – the geriatric puppet characters created by Albion that had been criticised for appealing to the young and vulnerable – in any future campaigns.
A spokesman for Wonga said: "As part of the significant changes we’re making at Wonga, we’ve started the search for a new creative agency.
"We’re committed to building a responsible, sustainable business and to communicating with the right customers in the right way."