Bacs Direct Debit

Bacs Direct Debit is used by the majority of UK recurring billing programs — utilities, telecoms, subscription SaaS, insurance, and B2B invoice collection. Unlike ACH, Bacs has no same-day settlement option; its fixed three-day submission cycle is a structural property of the scheme. Understanding the Service User Number structure, the three-day cycle, and the Direct Debit Guarantee is the operational baseline for any operator building UK bank-account billing.

Service User Number (SUN)

Every Bacs Direct Debit originates under a Service User Number — a six-digit identifier linking all Direct Debit Instructions (DDIs) to the operator. Operators obtain a SUN in one of two ways:

Directly from a sponsoring bank: Available to high-volume originators meeting the bank’s risk and volume requirements. The operator holds the SUN; mandates are portable within that SUN relationship.

Through a Bacs-approved bureau: The bureau holds the SUN; the operator is a client. GoCardless and Bottomline Technologies are common UK bureaux. Bureau-obtained mandates are registered under the bureau’s SUN and cannot be transferred to another bureau without customer re-authorization. This mandate portability constraint is a meaningful churn risk for operators considering a bureau migration.

The three-day submission cycle

Bacs processes on a fixed three-business-day cycle from file submission to settlement:

Collections must be scheduled at least three business days ahead of the intended payment date. There is no accelerated settlement option for Bacs debits. Operators managing time-sensitive billing events — end-of-trial conversions, invoice due dates — must build this lead time into their collection scheduling.

Direct Debit Guarantee

The Direct Debit Guarantee applies to every Bacs Direct Debit in the UK and provides three protections to the payer:

1. Advance notice of amount and date before any collection is applied
2. Immediate full refund from the payer’s bank for any Direct Debit applied incorrectly — wrong amount, wrong date, or unauthorized
3. Cancellation right at any time by notifying the bank

There is no fixed time limit on unauthorized-debit claims under the Guarantee. A payer can claim a refund for a payment they assert was never authorized at any point, creating ongoing liability for operators with mandate management gaps.

Indemnity claims

When a payer exercises the Guarantee, their bank pays the refund immediately and recovers the funds from the operator’s sponsoring bank or bureau via an indemnity claim. The operator’s account is debited. The operator can dispute the claim with evidence of a valid DDI, but the default outcome favors the payer. High indemnity claim rates are a compliance signal that can result in SUN review or suspension.

AUDDIS

AUDDIS (Automated Direct Debit Instruction Service) is the electronic system for submitting, amending, and cancelling DDIs without paper forms. Operators submit DDI data electronically via their sponsoring bank or bureau; the payer’s bank validates the account before the first collection is attempted, reducing initial failure rates. AUDDIS is the standard approach for any operator using a bureau or modern payment provider. Under AUDDIS, the first collection must occur at least three working days after DDI submission.