Sweden’s defence minister has demanded that Denmark explain why its foreign secret service allegedly helped the US spy on European leaders, including German Chancellor Angela Merkel, more than seven years ago.

“We want the cards on the table,” said defence minister Peter Hulqvist, who added it was “unacceptable to eavesdrop on allies”.

Danish legislator Karsten Hoenge, of the left-leaning Socialist People’s Party that is supporting Denmark’s Social Democratic government, said he would quiz the Scandinavian country’s defence and justice ministers in parliament about the case.

“The government must explain how come Denmark has been acting as a willing tool for a US intelligence service, and what it will mean for co-operation with Denmark’s neighbouring countries,” he said.

The Danish broadcaster DR said the Danish Defence Intelligence Service, known in Denmark by its acronym FE, conducted in 2014 an internal investigation on whether the US National Security Agency (NSA) had used the co-operation with the Danes to spy against Denmark and neighbouring countries.

The probe, codenamed “Operation Dunhammer,” concluded that the NSA had eavesdropped on political leaders and officials in Germany, France, Sweden and Norway.

According to DR, the alleged set-up between the United States and Denmark reportedly allowed the NSA to obtain data by using the telephone numbers of politicians as search parameters.

DR said its report was based on interviews with nine unnamed sources, all of whom were said to have had access to classified information held by the FE.

The military agency allegedly helped the NSA from 2012 to 2014.

Reports in 2013 that the NSA listened in on German government phones, including Mrs Merkel’s, prompted a diplomatic spat between Berlin and Washington that soured otherwise good relations with Barack Obama’s administration.

Mrs Merkel declared that “spying among friends” was unacceptable and said later that she stood by that comment.

Still, there were also reports that Germany’s own BND intelligence agency may have helped the US spy on European companies and officials.

In a written comment to DR, Danish defence minister Trine Bramsen said the government cannot discuss intelligence matters.

She added the present government has “the same point of view” as the former Social Democratic government led by Helle Thorning-Schmidt, who was in power during that period: “the systematic wiretapping of close allies is unacceptable”.