May.30, 2010, 9:54 AM

Peter Mandelson’s controversial friend Oleg Deripaska is being treated as a formal suspect in a money-laundering case linked to the Russian mafia.

The Russian oligarch, who has entertained Lord Mandelson and Chancellor George Osborne on his luxury yacht, has been warned that he is a suspect at meetings with Spanish prosecutors in Moscow.

They questioned the billionaire about his alleged involvement in laundering £4.5million used to buy Spanish property via companies he controlled.

During taped testimony, Mr Deripaska, the chairman of the world’s biggest aluminium company, Rusal, reportedly admitted having contacts with the Russian mafia, saying he was forced to pay them protection money.

But he denied any involvement in criminal activity.

The Spanish state prosecutor’s office has confirmed to The Mail on Sunday that Mr Deripaska was interrogated as a suspected money launderer by an investigating judge from the country’s National Criminal Court earlier this month.

The interview was conducted in Moscow and the oligarch was cautioned by Judge Fernando Andreu that he was being interrogated as what in Spanish is known as an ‘imputado’.

The term is used when there are indications that a person has possibly committed a criminal offence. Such a person has the right to refuse to answer questions and the interrogation by the judge has to be conducted in the presence of a lawyer representing the person being questioned.

Fernando Noya, a spokesman for Jose Grinda, the state prosecutor who attended the hearing in Moscow, confirmed: ‘Mr Deripaska was interrogated as an imputado for suspected laundering of capital in Spain and elsewhere and criminal association.’

It is unclear how Mr Deripaska reacted to the allegation. Last night he issued a statement through his London spokesman. It read: ‘Mr Deripaska was happy to answer the questions of the Spanish legal authorities during their visit to Moscow to assist in their investigation.

‘Mr Deripaska intends to observe the confidential nature of the discussions and has no intention of making further comment other than to reiterate that has had no involvement whatsoever in the activities that are the subject of the Spanish investigation.’

Court orders signed by the Spanish judge show that the investigation has requested thousands of pages of evidence compiled for a case involving Mr Deripaska in the High Court in London.

In that case, Michael Cherney is suing Mr Deripaska for enforcement of contracts they signed in March 2001. Mr Cherney claims he is owed a 13 per cent stake in Rusal.

In the British court Mr Deripaska has denied that he was Cherney’s partner, or that he owes him shares or money.

The Spanish court has claimed that Mr Cherney is one of Mr Deripaska’s partners and an alleged co-conspirator.

Lord Mandelson, the former Business Secretary, has met Mr Deripaska on at least five separate occasions, staying on his luxury yacht in Corfu.

The case will also reignite questions over Mr Deripaska’s links to Mr Osborne, who was forced to deny asking him for a donation to the Tory Party.