The European Investigative Collaborations consortium, a group working alongside several newspapers including the Sunday Times, claims to have leaked data about ‘murky financial transactions’ and ‘tax tricks’ used by a number of managers and players in European professional football. This is said to embrace 18.6m documents, including original contracts with secret subsidiary agreements.

According to the consortium Mourinho is said to have avoided paying tax on earnings from the use of his image rights for product endorsement by moving £10m offshore into a Swiss account owned by a British Virgin Islands (BVI) firm. Substantial costs were deducted for the BVI company although, it is alleged, it had no employees.

There are also claims that Real Madrid player Cristiano Ronaldo allegedly moved money to the BVI as part of highly aggressive tax avoidance’, and that both player and manager used bank accounts and companies in Ireland and New Zealand to process substantial earnings for their image rights.

The so-called ‘Football Leaks’  allegations concern Mourinho’s spell as manager of Chelsea, between 2004 and 2007, and the Spanish club Real Madrid, between 2010 and 2013.

Responding to the reports, Hillier said: ‘These revelations are extraordinary and warrant a close examination by the UK tax authorities.’

Gestifute, a company which has advised a number of sporting professionals including those named in the reports, has rejected the allegations.

In a statement the company said: ‘One newspaper makes the unfounded allegation that Gestifute, its clients and its advisers of having, directly or indirectly, created, organised or designed tax structures to facilitate tax evasion using tax havens. The most serious and malicious allegations in the inquiries relate to José Mourinho and Cristiano Ronaldo.

‘Both Cristiano Ronaldo and José Mourinho are fully compliant with their tax obligations with the Spanish and British tax authorities. Neither Cristiano Ronaldo nor José Mourinho have ever been involved in legal proceedings regarding the commission of a tax offence.’

An HMRC spokesman said: ‘HMRC carefully scrutinises the arrangements between football clubs and their employees in respect of any image right payments to make sure the right tax is paid – in recent years we have identified more than £80m in additional tax payable from clubs, players and agents.

‘We take seriously allegations that customers or their agents may have acted dishonestly in the course of an enquiry, and can reopen closed cases if we suspect this has happened.’

HMRC also stated: ‘We don’t talk about named individuals. We take all allegation of tax evasion extremely seriously and always investigate allegations of fraud together with any intelligence provided.’