The anonymous source behind the leak of the Panama Papers has spoken for the first time, offering to help law authorities make prosecutions in return for immunity, BBC News reports.

In a 1,800-word statement, "John Doe" reveals he has never worked for a spy agency or a government. He starts the statement by citing "income equality" as a motive.

The Panama Papers have shown how some wealthy people use offshore firms to evade tax and avoid sanctions.

The papers belonged to the Mossack Fonseca law firm. It denies any wrongdoing and says it is the victim of a hack.

The papers were investigated by hundreds of investigative journalists, including from the BBC, who worked in secret with the International Consortium of Investigative Journalists (ICIJ) for months.

The documents have revealed the hidden assets of hundreds of politicians, officials, current and former national leaders, celebrities and sports stars.

They list more than 200,000 shell companies, foundations and trusts set up in tax havens around the world.

The John Doe statement came shortly before US President Barack Obama delivered an address on the economy, in which he cited the Panama Papers as highlighting the problem of corruption and tax evasion.

He said the US would require banks to identify those behind shell corporations. Mr Obama said his administration's actions would allow it to do a better job of making sure people paid taxes.