People need to think carefully about permissions around shared bank accounts after a stream of complaints, Banking Ombudsman Nicola Sladden says.
Sladden said her call was particularly directed at companies, trusts, partnerships and charities where a handful of individuals might operate a single account.
She said account-holders need to be clear whether they want authorised signatories to be able to act alone, or only together.
"Account mandates tell banks who can use an account, but also how those individuals can use it. Selecting the right wording makes all the difference to the 'how' because banks are obliged to act exactly as directed," Sladden said.
"Allowing 'any' or 'either' authorised signatory to operate an account (or operate it 'severally') will mean one person can do so on his or her own.
"Specifying 'joint', 'both', 'all' or 'any two' authorised signatures will mean no one on his or her own can withdraw or transfer funds."
Sladden said establishing a clear mandate could avoid costly trouble down the track, such as misuse of funds by someone authorised to act alone.
The Banking Ombudsman Scheme has published a quick guide to account mandates.