More than 70 politicians, academics, union heads and charity leaders around the world have come out in opposition to the decision by Luxembourg to prosecute the 28-year-old accountant accused of sparking the LuxLeaks tax scandal.

Antoine Deltour, who spent two years as a junior auditor at PricewaterhouseCoopers before quitting in 2010, was this month charged with a string of criminal offences. He has said: “From the beginning, I acted out of conviction for my ideas, not to appear in the media.”

The charges came less than six weeks after the Guardian and more than 20 news organisations around the world, in conjunction with the International Consortium of Investigative Journalists (ICIJ), published detailed investigations into the tax affairs of several multinationals based on leaked tax rulings that PwC had secured in Luxembourg for some of its clients.

In an open letter to prosecutors in the Grand Duchy, critics of the decision to prosecute Deltour argued that the leak had been “manifestly in the public interest, helping to expose the industrial scale on which Luxembourg has sanctioned aggressive tax avoidance schemes, draining huge sums from public coffers beyond its borders”.

Deltour is charged with theft, violating Luxembourg’s professional secrecy laws, violation of trade secrets, and illegally accessing a database. The charges stem from an official complaint brought by PwC. He could face jail and a heavy fine.

The Grand Duchy is reeling from what finance minister Pierre Gramegna has called “the worst attack Luxembourg has experienced in its history”.

Luxembourg tax rulings granted to subsidiaries of Amazon and Fiat were already under investigation by the European commission, suspected of being sweetheart deals amounting to illegal state aid.

For months, Luxembourg has refused to hand over further tax ruling information sought by the commission, disputing the legality of such requests. Last week, however, ministers performed a U-turn in the wake of the scandal, agreeing to provide documents sought by state aid investigators so long as similar requests were made of other EU member states.

Explaining the volte-face, Gramegna described the LuxLeaks affair as a “game changer” that had transformed the way European regulators were scrutinising tax rulings granted to multinationals.

Politicians from Germany, France, the UK, the US and Australia were among the signatories to the letter opposing the prosecution of Deltour, which was organised by a handful of media groups including the Guardian. The majority of political signatories were from parties of the left, but there was also support from among Liberal Democrats in the UK and from the centre-right UMP in France, led by Nicolas Sarkozy.

US Congressman Lloyd Doggett, a member of the ways and means committee that has held a number of hearings on tax avoidance, also signed the letter. So too did his fellow Democrat member of Congress Rosa DeLauro. In the UK the letter was signed by four MPs, including the former environment minister Michael Meacher and two members of the public accounts committee, which this month summoned PwC’s head of tax in the UK to face a grilling on the Luxembourg situation. Among UK union leaders supporting the letter were Unite’s Len McCluskey, Paul Kenny of the GMB, and Mark Serwotka, general secretary of the Public and Communications Services union.

In Australia signatories included Ged Kearney, president of the Australian Council of Trade Unions, and Rev Prof Andrew Dutney, president of the Uniting Church in Australia. Senior figures from a host of charities and campaign groups supported the letter, including War on Want, ONE, Oxfam Novib, Action Aid, Christian Aid, Justice et Paiz, Eurodad, Transparency International, Christian Aid, Global Financial Integrity, the Fact Coalition and the Tax Justice Network.