Redknapp is convinced the Championship club should be allowed to play in Europe - but only if they defeat his side in the Wembley showpiece.
"If Cardiff beat us, I will support them 100 million per cent in their bid for European football," he told the Portsmouth News.
"I'd be first in the queue. If they win the FA Cup, they should be in Europe - what's wrong with that?
"Cardiff have been in the English League for 80-odd years. If they beat us, I will shake (manager) Dave Jones' hand and wish them the very best .
"They would have deserved the prize of European football and the very best to them.
"It's a question that beats me, but what is the point of them entering in the first place, then?"