Credit card company MBNA has been forced to improve the way it deals with customers struggling with debts after intervention from a regulator.
The move comes after the Office of Fair Trading (OFT) investigated the firm's in-house debt collection arm.
The review found that customers did not know if partial payments to start paying back debts had been accepted, and MBNA had bypassed debt advisers.
MBNA said it had agreed to "greater transparency".
The investigation began after Citizens Advice raised concerns with the OFT. It was worried about how MBNA dealt with customers in financial difficulty.
As a result of the investigation, the lender agreed that - from January - it would make it clear in letters to customers whether it would accept repayments at a level they could afford.
It would also ensure that it would deal with a debt adviser appointed to act on a customer's behalf, instead of bypassing this representative and dealing with the customer directly.
Direct contact would only occur if the adviser had given permission, they no longer acted for the customer, or a payment had not been made and the representative could not be contacted.
"Our investigation found problems with the way MBNA communicates with customers in financial difficulties," said Ray Watson, director of the OFT's consumer credit group.
If MBNA fails to keep to the new agreement, it can be fined up to £50,000 per breach.
A spokesman for MBNA said: "As a result of ongoing dialogue with the OFT we have agreed to deliver greater transparency in some of the communications we have with our customers in financial difficulties.
"We believe these changes are enhancements to our existing strong practices. We are in the process of implementing them now."