The family of Kenya's President Uhuru Kenyatta, that has dominated the country's politics since independence, secretly owned a network of offshore companies for decades, according to a huge leak of financial papers.

The Pandora Papers - 12 million files - is the biggest such leak in history.

Mr Kenyatta and six members of his family have been linked to 13 offshore companies.

They have not yet responded to requests for comment.

The Kenyattas' offshore investments, including a company with stocks and bonds worth $30m (£22m), were discovered among hundreds of thousands of pages of administrative paperwork from the archives of 14 law firms and service providers in Panama and the British Virgin Islands (BVI) and other tax havens.

The secret assets were uncovered by an investigation, published earlier on Sunday, by the International Consortium of Investigative Journalists (ICIJ), Finance Uncovered, Finance Uncovered, Africa Uncensored and other news organisations. 

Documents show that a foundation called Varies was set up in 2003 in Panama, naming Mr Kenyatta's mother, Ngina, 88, as the first benefactor - and Kenya's leader as the second benefactor, who would inherit it after her death.

The purpose of the foundation and the value of its assets are unknown.

Panamanian foundations are much sought after because the true owners of the assets are only known by their lawyers and they do not have to register their names with the Panamanian government, ICIJ reports.

The assets can also be designed to be transferred tax-free to a successor.

There's no reliable estimate of the Kenyatta family's net worth but its vast business interests span transport, insurance, hotels, farming, land ownership and the media industry in Kenya.

In 2018, Mr Kenyatta told the BBC Hardtalk programme that his family's wealth was known to the public, and as president he had declared his assets as required by law. 

"As I have always stated, what we own - what we have - is open to the public. As a public servant I'm supposed to make my wealth known and we declare every year," Mr Kenyatta said.

"If there's an instance where somebody can say that what we have done or obtained has not been legitimate, say so - we are ready to face any court," he added.

In the same interview, Mr Kenyatta said he wanted fighting corruption and promoting transparency to be his legacy.

He promised to work with parliament to create a law that would oblige public officials to declare their wealth, but MPs are yet to pass this bill.

Other world leaders named in the Pandora Papers include the King of Jordan Abdullah II, former UK Prime Minister Tony Blair, Gabon's President Ali Bongo Ondimba and President of Congo-Brazzaville Denis Sassou-Nguesso.

It is unclear if President Kenyatta, who retires next year after 10 years in office, knew about the Varies foundation but the timing of its opening may be instructive. 

Seven months earlier, he had lost the 2002 presidential election to opposition candidate Mwai Kibaki, who had vowed to redress historical crimes as well as launch a war against corruption.

At the time, the family of outgoing president Daniel arap Moi, a friend of the Kenyattas, allegedly moved money out of the country, according to a 2014 leaked report by the international risk consultancy Kroll.