An Australian millionaire socialite and her daughter are unable to provide any "relevant assistance'' in the search for Madeleine McCann, investigators said.

Reports had linked a luxury yacht owned by Rhonda Wyllie and Melissa Karlson, the widow and daughter of the late Perth property tycoon Bill Wyllie, to the inquiry.

The boat was moored at a marina in Barcelona, Spain, where a woman with an Australian accent reportedly asked a bystander if he was there to deliver her "new daughter'' days after Madeleine, of Rothley, disappeared.

But Mrs Wyllie and Ms Karlson insisted they were in Antibes, France, on the night in question and said they could not help the investigators.

They said they would co-operate with the police to confirm all the facts.