Secret tax deals between Luxembourg and multinationals have increased dramatically since the LuxLeaks scandal broke late 2014, according to a new report.

Eurodad, a Brussels-based NGO, found that so-called sweetheart deals, elaborate schemes used to slash corporate global tax bills, increased by 50 percent in Luxembourg during the year following the scandal.

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The report, released on Wednesday (7 December), analysed data from 17 EU states and Norway.

Tove Maria Ryding, one of the author's behind the 124-page report, said in a statement it's "as if the LuxLeaks scandal never happened."

The November 2014 scandal revealed that Luxembourg had signed off secret deals with some 340 corporations from 2002 until 2010.

Billions of taxes had been diverted away from national coffers and back into corporate pockets. Some firms managed to get away with effective tax rates of less than 1 percent on profits.

The Grand Duchy, once presided over by EU commission president Jean-Claude Juncker, also remains one of the most secretive countries in the world.

Another investigation in April linked some 10,000 offshore accounts to Luxembourg, following a massive leak of documents from the Mossack Fonseca law firm in Panama.

The LuxLeaks scandal broke when Juncker took the top post at the EU Commission. Juncker has consistently denied any wrongdoing despite having governed the small landlocked nation for two decades.

“What comes under the term LuxLeaks, really what they are talking about is a kind of common practice in many member states. In fact, that is why I’d rather say EUleaks than Luxleaks," he told MEPs in September last year.

Juncker's description appears prescient given that the recent spike in such deals is not exclusive to Luxembourg. Belgium's tax deals increased by 248 percent in only one year.

The report notes that the number of sweetheart deals in the EU went from 547 in 2013, to 972 in 2014, and reached 1,444 by the end of 2015 or increase of 160 percent in just two years.

"The fact that multinational corporations now have more than 1,000 sweetheart deals in Europe is deeply concerning, to say the least," noted Ryding.

One study by the EU parliament in 2015 said such deals cost the EU up to €70 billion in lost tax revenue every year.

The EU commission has since launched probes into big tax dodgers.

Earlier this year they ruled Apple had received €13 billion in illegal corporate tax benefits from Ireland. The order triggered an immediate backlash from Apple and the governments in Ireland and the United States.

It's also looking into Luxembourg linked firms like Amazon, Engie [formally GDF Suez], and McDonald's.

It had also launched anti-tax avoidance laws earlier this year but the rules were later weakened by member states.

Tax transparency advocates are instead pushing to make companies financial information public. The idea has gained some traction with more countries now backing plans, when compared to last year, of creating public registers that reveal the true owners of businesses.