Belgian police could have saved the lives of two children allegedly murdered by the paedophile Marc Dutroux if they had watched a video seized from his home which showed him building their hidden cell, it emerged yesterday.

The force admitted that it had held on to the footage for nearly four years without passing it to the judges investigating the country's paedophile ring. Prosecutors were given copies of the tape earlier in their inquiry with the vital scene, and footage of Dutroux sexually assaulting a woman, edited out.

The justice minister, Tony van Parys, said yesterday that he was "truly scandalised" by the disclosure. The judge in charge of the Dutroux inquiry, Anne Thily, demanded an internal police investigation.

Jean-Denis Lejeune, father of one of the two eight-year-old girls Dutroux is alleged to have allowed to starve to death while hidden under his house in Charleroi, said: "I just don't understand anything anymore. We don't have any confidence in the inquiry, and that doesn't date just from now."

Dutroux, a builder, was arrested in August 1996, when the bodies of Julie Lejeune and her friend Melissa Russo were found buried in his back garden. He is charged with abducting them 14 months earlier. It is alleged that they died the previous winter, locked in the cell, because Dutroux's wife Michelle Martin, a primary school teacher, did not feed them while her husband was serving a four-month jail term for other offences.

Police looking for the girls searched Dutroux's house three times, even hearing children's voices on one occasion, but failed to find the cell because it was hidden behind shelving. They had been told by a released convict that Dutroux had boasted about his plans to build a cell in his basement.

When Dutroux showed police the cell after his arrest, they found two other missing girls alive inside.

The video, apparently shot by Ms Martin, was seized in December 1995 while Julie and Melissa were still alive.

Yesterday's revelation came as the police took Dutroux, Ms Martin and their former lodger Michel Lelievre back to the house.

Officers said they wanted to test Ms Martin's claim that the door to the cell was too heavy for her to move.

The investigation is nowhere near a conclusion. Prosecutors expect to complete their case against the three accused early next year but the trial is not expected until the spring of 2001.