I don't recognise a card purchase

First, check it really is a stranger’s. Ask whoever holds the card. For an agent’s card, check what the agent was asked to buy. Merchants sometimes appear under a different name from their shop’s. If the purchase is yours but something went wrong, see Dispute a card purchase instead.

Who can do this: You need to be an owner, admin or manager to do this.

Find the purchase under Recent purchases on Cards, or on the card’s page.
Choose I don’t recognise this beside it.
Read what will happen, then choose Dispute and replace the card. The dispute states, for you, that nobody at your business made the purchase and you don’t know who did.

What happens straight away

The card is cancelled. Nobody can use it again. The card networks require this: while the card stays active, later fraud disputes on it would be lost.
A replacement is issued with the same rules, agent and limits, so the card’s work carries on. A physical card’s replacement is posted to you. A single-use card is cancelled, not replaced. Replacing a card for fraud is always allowed, even when you’ve reached your card limits.
The dispute is sent to the card network, which asks the merchant to respond. You don’t need to send anything else.

To follow the dispute later, go to Cards, then Card disputes.

The money

For a fraud dispute, the card network usually adds the disputed amount back to your card balance by the next business day while it decides. On a new account, that money is on hold until the dispute is decided. On an established one, you can spend it at once. See Why is my dispute money on hold?.

If you win, the money is yours. Tuppence pays the card network’s dispute fee, not you.

Don’t wait too long

You can usually dispute a purchase for 110 days after it was paid. Dispute as soon as you notice. If an agent’s card was used, also check who has its key. See Give an agent a key.

Your card limits never stop you disputing a purchase.