Denmark's secret service helped the US spy on European politicians including German Chancellor Angela Merkel from 2012 to 2014, Danish media say.
The Defence Intelligence Service (FE) collaborated with the US National Security Agency (NSA) to gather information, according to Danish public service broadcaster DR.
Intelligence was allegedly collected on other officials from Germany, France, Sweden and Norway.
Similar allegations emerged in 2013.
Then, secrets leaked by US whistleblower Edward Snowden alleged tapping of the German chancellor's phone by the NSA.
When those allegations were made, the White House gave no outright denial but said Mrs Merkel's phone was not being bugged at the time and would not be in future.
Germany is a close ally of the US.
German President Frank-Walter Steinmeier and a spokesperson for Angela Merkel have said they were not aware of Danish involvement until the DR report, which was shared with other European media over the weekend.
The NSA is said to have accessed text messages and the phone conversations of a number of prominent individuals by tapping into Danish internet cables in co-operation with the FE.
The alleged set-up, said in the report to have been codenamed "Operation Dunhammer", allowed the NSA to obtain data using the telephone numbers of politicians as search parameters, according to DR.
DR interviewed nine sources, all of whom are said to have had access to classified information held by the FE.
Along with Mrs Merkel, then-German Foreign Minister Frank-Walter Steinmeier and the opposition leader at the time, Peer Steinbrück, are also said to have been targeted.
"Politically I view this as a scandal," Mr Steinbrück told German media.
Denmark's Defence Minister Trine Bramsen, who had reportedly been earlier informed of the espionage, told DR that "systematic wiretapping of close allies is unacceptable".
Other European politicians have condemned the reports.
Norwegian Prime Minister Erna Solberg told public broadcaster NRK: "It's unacceptable if countries which have close allied co-operation feel the need to spy on one another."
France's Europe Minister Clement Beaune told France Info radio the allegations were "extremely serious".
Neither the FE nor the NSA have yet commented on the latest reports.
Following the new report, Mr Snowden accused US President Joe Biden of being "deeply involved in this scandal the first time around". Mr Biden was US vice-president at the time the reported surveillance took place.
"There should be an explicit requirement for full public disclosure not only from Denmark, but their senior partner as well," he tweeted.