UPDATE: April 5th 1230 GMT: This article has been updated to include the resignation of Sigmundur David Gunnlaugsson, Iceland's prime minister.

WITHIN a day of the first articles appearing, the worldwide fallout from a massive leak of financial documents, dubbed the “Panama papers”, was already huge, with several countries announcing that they would investigate suspected tax evasion and other wrongdoing. The journalists who have been combing through the trove allege that around 140 political figures, including Russia’s president, Vladimir Putin, and his Chinese counterpart, Xi Jinping, have links to the holders of the offshore accounts through family members, friends or associates. More revelations are expected this week. The furore will only grow.

On April 3rd a group of investigative journalists from 78 countries began publishing stories exposing the hidden wealth of national leaders, state officials, celebrities and others, based on emails and other documents from the database of a Panamanian law firm, Mossack Fonseca. These files date back several decades, but the most recent are only a few months old. They contain details of secretive offshore structures used by Mossack’s clients. The British Virgin Islands is the most popular domicile for their anonymous shell companies, with Panama in second place.

The files were leaked to Süddeutsche Zeitung, a German newspaper, which shared them with the International Consortium of Investigative Journalists. The ICIJ has already made several splashes with leaked documents, including the “Lux leaks” files, which exposed sweetheart tax-avoidance deals between multinationals and the government of Luxembourg. But none has been anywhere near as big as this one, or as likely to ensnare public figures.

The uses to which these shell companies, trusts and the like are put range from the perfectly legitimate to tax evasion and, further along the spectrum of miscreancy, the suspected looting of public money. (Mossack has responded to the leak by pointing out that it has never been charged with wrongdoing and insisting it conducts thorough due diligence on those with whom it does business.)

As much as $2 billion of loans and investments has flowed through offshore entities linked to friends and associates of Mr Putin, according to the ICIJ’s reporting. Among these is Sergei Roldugin, a cellist who is a close friend of the president and godfather to one of his daughters. One of the investments part-owned by Mr Roldugin is a 12.5% stake in Russia’s once largest television-advertising agency (which has reportedly never disclosed its ownership); another is 3.9% of Bank Rossiya, on which America imposed sanctions after Russia invaded Ukraine in 2014. American officials have described Rossiya as Mr Putin’s personal bank. A spokesman for Mr Putin said on April 4th that there was nothing much new in the reporting on the Panama files, and the claims were an example of “Putinophobia”.

On April 5th, Iceland’s prime minister, Sigmundur David Gunnlaugsson, resigned: the files show that he transferred his half share in an offshore company to his wife for the token sum of $1. He has denied allegations that the vehicle was used to hide a large investment in Icelandic banks. Mr Gunnlaugsson says he is being subjected to a “witch-hunt”.

Several countries, including Sweden, France and Australia, have already opened investigations. Australia’s tax authority is reportedly probing more than 800 clients of Mossack Fonseca.

The leak will greatly add to pressure on Panama to adopt emerging global standards on the exchange of tax information, and to enforce new anti-money-laundering laws properly. Among significant tax havens, the Central American country has been one of the slowest to embrace change.

More broadly, the revelations will inject some urgency into a global anti-corruption summit, which is to be held in London in May. David Cameron, Britain’s prime minister, rightly sees murky ownership of offshore shell companies as facilitating financial malfeasance. He has pushed through reforms that will make Britain the first big economy to open a public register that reveals the ultimate beneficial ownership of such outfits later this year. Unfortunately the idea has been slow to catch on. The Panama leak shows that lax or complicit gatekeepers—the law firms, corporate service providers and accountants that set up and oversee offshore companies—are another big problem.

For those hoping for change, the good news is that, as the public mood towards offshore financial shenanigans dims, more and more employees of financial firms and law firms seem willing to slip hard drives packed with data pointing to dodgy goings-on to the media or other watchdogs. Doing so may break the law, but the justification is that it sometimes takes an illegal act to expose an illegal act. Leaks may be the most powerful weapon in the war on financial secrecy.

Dig deeper:The Economist explains why the Panama papers matter