A European Court of Justice ruling could deal a “serious blow” to Theresa May’s most prized piece of legislation, campaigners have said.

The new Prime Minister’s time in her previous job as Home Secretary was arguably defined by her oversight of the Investigatory Powers Bill – which hands over vastly more spying powers to police and other agencies, and forces internet companies to store information about users’ entire browsing history. But that bill could have been dealt a serious blow by a case brought by a member of her own cabinet.

The European Court of Justice heard a case against the government’s powers to store data on their own citizens, arguing that such bulk collection is not legal. And though the court’s advocate general said that such storage might be legal – its findings could still cause huge problems for Theresa May’s spying powers.

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The case had originally been brought by a group including David Davis, the Conservative MP who has campaigned on privacy issues. But he dropped off the case in the last week, soon after he re-joined the cabinet.

Campaign group Privacy International said that the opinion is a “serious blow” to the bill, and that the full judgement from the court is likely to cause further problems.

“The bulk powers - what we would call mass surveillance powers - embedded throughout the IPBill go far beyond tackling serious crime,” said Privacy International’s general counsel, Caroline Wilson Palow. “They would give a range of public bodies, not just the Police and intelligence agencies, the power to access the personal data of innocent people, often without any form of warrant.

“The Advocate General's opinion supports our calls for much stricter safeguards and oversight to protect us from serious violations of our privacy - including that all access to our data, including communications data, must be authorised by an independent authority such as a judge.”

The group said that the opinion should serve as “a wake up call for lawmakers that the IPBill's powers and safeguards need to be overhauled."

Green Party peers agreed that the opinion should lead lawmakers to rewrite major parts of the bill.

“Today’s European Court of Justice ruling - that bulk data collection is only lawful if it is used to tackle serious crime - makes it clearer than ever that the Investigatory Powers Bill currently passing through the House of Lords is simply not fit for purpose,” said Baroness Jones of Moulsecoomb, who represents the Green Party in the House of Lords.