You do not require a credit card to confirm the reservation.
You do require a credit card when the car is picked up. You do require a credit card present and the credit card owner present to sign the Rental Agreement when collecting the car. The terms and conditions on the booking page will display the rental requirements of your supplier - each vary slightly. Please note, pre-paid credit cards will not be accepted, ie. Bopo cards.
Debit Cards with Visa or Mastercard emblem.
Avis and Hertz will accept debit cards (not pre-paid cards) with the Visa/Mastercard emblem at most locations. This excludes any luxury fleet bookings.
Budget and Thrifty will accept debit cards (not prepaid) with the Visa/Mastercard emblem embossed on them at most major locations. This excludes luxury fleet bookings and may require an additional bond upfront.
Europcar will accept debit card in limited circumstances on some standard passenger vehicles (not luxury, specialty or vans). Payment by debit card requires a bond to be paid at the start of the rental, and refunded on return. Europcar states if payment is by Debit card, the renter must also reduce their loss damage waiver.
Always refer to the supplier terms and conditions - to clarify their requirements.