Even without an arranged overdraft, your balance can still go below £0 (essentially a negative balance) - this is called an unarranged overdraft.

We'll usually reject any payments that would take you into an unarranged overdraft. But unfortunately, we can’t reject all payments - some transactions can happen when payments are offline transactions or delayed (e.g TfL payments).

• Our unarranged overdraft rate is the same as our arranged overdraft rate (24.9% APR).

• The monthly maximum charge for interest on an unarranged overdraft is capped at £15.

More information can be found here.