Jürgen Mossack said in a document, obtained by French news agency AFP, that the number of new tax shelters created has fallen by 30% in Panama and elsewhere after more than 11 million documents were leaked from the law firm in April 2016.
“However, jurisdictions such as Delaware, Nevada and others located in the United States, where virtually no due diligence is required […] incorporations are thriving!” Mossack said.
“While Panama tried hard to be whiter than white, others are profiting,” he wrote.
Mossack said US jurisdictions have “zero transparency” and that is why there has been a rise in the number of clients creating tax shelters in the US who, previously, would have gone to Panama.
Prior to the leak, Panama was the last major holdout, refusing to exchange banking information.
However, the country has since adopted new legislation and signed up to the international fight against tax fraud.
Mossack and his partner Ramon Fonseca were arrested in February and denied bail on money laundering charges as part of a Brazilian corruption probe.
His letter, dated 10 April, was written from detention, AFP reports.
According to him, “less than 1% of all the companies incorporated” by Mossack Fonseca could have “a possible wrongful use”.
If some of the companies set up by the law firm were used by their owners to try to cheat on their taxes, “we would not have been aware of that, since no client in his right mind would tell people he has never met that he would use the company for an illegal purpose.
“Such a disclosure would automatically disqualify him from being accepted as a client,” Mossack said.
Biggest ever blow
Mossack Fonseca, which specialises in setting up offshore companies in tax havens, was behind the leak, which the International Consortium of Investigative Journalists (ICIJ) called “the biggest blow the offshore world has ever taken”.
Following a year-long investigation, emails, bank records and client documents spanning from 1977 to 2015 were leaked to German newspaper Süddeutsche Zeitung.
They were then shared with the ICIJ, as well as BBC Panorama and the Guardian – both based in the UK.
The papers revealed how rich and powerful people around the world set up offshore trusts in a bid to avoid sanctions and evade tax.
The international law firm has denied wrongdoing and said it was the victim of a computer hack.
It also maintains that the information leaked was being misrepresented.