The US case against Megaupload founder Kim Dotcom is looking shakier by the day, following news that warrants used to raid his New Zealand home were invalid.

On Thursday New Zealand High Court Judge Justice Helen Winkelmann ruled that the warrants used in January “did not adequately describe the offences to which they related”. Many of Dotcom’s substantial assets were frozen, and property seized including a 20 luxury vehicles, works of art and computers.

Moves by the FBI to copy data from Dotcom’s computer and take it offshore were also unlawful, she said.

In January Dotcom was cut out of a safe room in his $30m Aukland mansion by SWAT officers and arrested along with four others. Now under house arrest, he was initially judged a flight risk and denied bail.

Megaupload was unceremoniously executed, provoking outrage from millions of users who had legitimately used the service to …