Gerry McCann issued a stark warning on the steps of a Lisbon court by saying the people who snatched his daughter Madeleine could strike again.

McCann spoke out after hearing his wife Kate tell a libel trial that their son Sean has told her "you hid Madeleine."

The couple were in Portugal as part of their defamation case against former police chief Goncalo Amaral, who wrote a banned book claiming the McCanns hid Madeleine's body after she was killed in an accident.

Gerry McCann insisted the real culprits remained at large.

"He or she or they may strike again," he said. "There's an unsolved serious crime and there's a series of other crimes against children which have come to light who have been on holiday so at the very least these people need to be brought to justice.

"We don't know if Madeleine is alive or dead but there is no evidence that she is dead and she is a missing child and she is completely innocent."

British police have stepped up efforts to find out what happened to Madeleine in Praia Da Luz. Teams from Scotland Yard have been in the Algarve town scouring a patch of scrubland and have also interviewed four 'persons of interest.'

Local opposition to the campaign has stepped up, with graffiti branding the police "stupid" amid fears tourists could be frightened away from the resorts.