EU judges may be asked to decide whether the intelligence services’ bulk collection of email data in order to prevent terrorist attacks is legal.

In a fresh challenge that could impact the Investigatory Powers Act, the campaign group Privacy International has argued in court on Monday that interception of social media that is not targeted and subject to sufficient safeguards is forbidden by a previous European judgment.

After a minute’s silence in memory of the victims of the attack in London on Saturday, the investigatory powers tribunal (IPT) said it was considering whether to refer the issue – concerning the use by GCHQ, MI5 and MI6 of bulk communication data – back to the court of justice of the European Union in Luxembourg.

“This case raises a fundamental political question as to the competing powers of the nation state and the EU,” the vice-president of the IPT, Mr Justice Mitting, told the hearing.

In December, the European court if justice ruled that “general and indiscriminate retention” of emails and electronic communications by governments is illegal. The far-reaching judgment was in relation to previous surveillance legislation but it may now effect the Investigatory Powers Act – the so-called snooper’s charter.

Only targeted interception of traffic and location data in order to combat serious crime – including terrorism – was justified, the EU’s highest court said.

The finding came in response to a legal challenge initially brought by the Brexit secretary, David Davis, when he was a backbench MP, and Tom Watson, Labour’s deputy leader, over the legality of GCHQ’s bulk interception of call records and online messages.

Addressing the IPT on Monday, Thomas de la Mare QC, for Privacy International, said: “Communications data collected in aggregate about individuals is no less sensitive than the content [of emails].”

UK citizens live in a country subject to a greater surveillance than any other, he said. “You should not forget that every single EU state has in the past five years been combatting Islamic terrorism of exactly the same form [as the Borough Market and Manchester attacks] and despite their efforts gross terrorist outrages have occurred ...

“Notwithstanding such terrible threats to human life, the constitutional right to personal privacy sets limits on state surveillance powers,” he said.

After the UK leaves the EU, the tribunal heard, European states may no longer be able to share intelligence with Britain unless the legal regimes are aligned.

James Eadie QC, for the government, has acknowledged to the tribunal in written submissions that the issue may have to be referred back to Luxembourg to clarify the EU court’s judgment.

His evidence also pointed out that the government faced “increasing use of encryption and increasing difficulty of interception” online that makes use of bulk communications data and bulk personal data more important.

Reliance on such bulk data surveillance reduces the need for more intrusive techniques, Eadie said. “The usefulness of bulk communications data is clear ... It provides more comprehensive coverage than is possible by means of interception. For example, it enables GCHQ to ‘tip off’ [MI5] when a subject of interest arrives in the UK,” he said.

The hearing is expected to last all week.