The Football Association has booked the Millennium Stadium for the 2006 FA Cup final in case Wembley is not ready.
The FA has always insisted that Wembley is on schedule but Millennium Stadium chief executive Paul Sergeant has confirmed that Cardiff is on standby.
"Saturday 13 May 2006 is firmly secured in our diary and we have signed an agreement to that effect," he said.
"The FA has every intention of staging the event at Wembley. It makes sense for them to have a back-up venue."
Sergeant added: "We wish all those involved in the construction of Wembley every success.
"These projects can be very complicated but it is in everyone's interests that the new stadium goes to plan.
"This agreement alleviates any pressure on the governing body should Wembley not be ready. This is a case of good planning and good football sense on the part of the FA."
Sergeant added that the fact the FA has signed the back-up deal with the Millennium Stadium was proof of the excellent job Cardiff has done in staging the final.
"This news is testament to the Millennium Stadium's success in hosting the past five FA Cup finals," he said.
"Football fans, and everyone involved in the game, can rest assured that we will deliver another fantastic day should we get the green light."
The FA announced last week that it was in negotiations to make sure that it had a contingency plan in place.
In a statement on their website last week, the FA said: "Multiplex are the expert contractors working to a fixed price contract.
"Therefore we have to be guided by their experienced advice that Wembley will be handed over to the FA by the end of March 2006 and be open for the FA Cup Final.
"However, it would be naive of any organisation involved in such a huge building project not to have alternative plans in place - should any unforeseen problems occur."