Labour wants Rishi Sunak to be more transparent about his finances after he was unable to answer questions about the source of hundreds of thousands of pounds he loaned to a company he shared with his wife.
The move has resulted in him being routinely referred to as the UK's richest MP, as he has been asked by the former chancellor to explain details about how he has managed his family's fortune, which is said to be 730 m.
The vast majority of the Conservative leadership candidate's wealth comes from his marriage to Akshata Murty, a member of the family that founded the Indian technology group Infosys, in which she has a stake worth of about 690 m.
Sunak has also worked in the hedge fund industry between 2006 and 2013, raising questions about whether the loans he personally made to his UK business came from profits generated in international tax havens.
James Murray, the shadow financial secretary to the Treasury, said Rishi Sunak wants to be our next prime minister, but again it seems that he is unable to give a straight answer to questions about his relationship with tax havens. It is hard to avoid the impression that he has something to hide. Sunak loaned 652,449 to Catamaran Ventures UK between 2013 and 2014, a company he shared with Murty at the time, according to documents filed at Companies House.
The loans were made shortly after Sunak had worked for the US branch of Theleme Partners in America between 2010 and 2013, where he managed three entities based in the US tax haven of Delaware.
Sunak was entitled to share of the profits made by one of the Delaware entities, US filings suggest, while industry sources said he was also likely to have invested his own money into the hedge fund.
Sunak said that Rishi has never used or benefited from a tax haven and that all of Theleme's US profits are subject to full US tax and that all profits within the group are taxed from an overseas bank account or from an overseas corporate account, which could potentially save Sunak taxes on bringing the money into the UK.
Sunak transferred his share of the UK company and the loan to his wife days before becoming MP in 2015, a transaction that would have been largely tax free.
There is no suggestion that Sunak has broken any tax laws and he has insisted that he has always been a normal UK taxpayer. However, the former chancellor has been embarrassed by his ties to people with less conventional tax arrangements.
Sunak and his family were criticised in April for avoiding 20 m in taxes because of Murty's non-dom status. She agreed to pay UK tax on her worldwide earnings in the future and for the last tax year after days of negative coverage, but she continues to be a non-domiciled citizen.
Murty has been revealed to be a shareholder in a restaurant business that funnelled investments through a company in the tax haven of Mauritius, a structure that could allow its backers to avoid taxes in India. The largest shareholder in the International Market Management IMM is Hugh Sloane, co-founder of the hedge fund Sloane Robinson, which was found to have operated a tax avoidance scheme by a tribunal in 2012.
In 2013, Sunak s boss at Theleme, Patrick Degorce, was forced to pay millions of pounds in tax after a ruling regarding a personal investment in a complex film financing scheme, which tried to hide earnings of almost 19 m.
A joint venture between Sunak's billionaire in-laws and Amazon was revealed last year to be involved in a multimillion-pound dispute with the Indian tax authorities.