In his interview with the Italian newspaper, the Archbishop denied the claims.
"The legal wedding was on the Saturday," he categorically stated.
Pressed for details on his meetings with the couple, Welby demurred, saying: "If any of you ever talk to a priest, you expect them to keep that talk confidential. It doesn't matter who I'm talking to."
He went on: "I had a number of private and pastoral meetings with the duke and duchess before the wedding.
"I signed the wedding certificate, which is a legal document, and I would have committed a serious criminal offence if I signed it knowing it was false. So you can make what you like about it. But the legal wedding was on the Saturday. But I won't say what happened at any other meetings."
The Archbishop's denial follows the royal couple's own backtracking of their earlier claims.
Through a spokesman, the couple told US news site The Daily Beast: "The couple exchanged personal vows a few days before their official/legal wedding on May 19."