CONVERT to GBP

Hi, If I have no USD and spend in the US will the card automatically debit form y Euro balance or GBP balance(which is my base currency) first?

Any help appreciated.

Please see this:
https://www.revolut.com/pl/help/exploring-revolut/spending-and-sending-money/using-my-card/how-will-my-currency-balances-be-deducted-for-spending?lang=en

As redi pointed out - in the mentioned scenario - it will first try to deduct it from sterling, then should there not be sufficient funds from euro.