A Rabbi who helped Roman Abramovich prove his Sephardi ancestry has been told he cannot leave the country in order to help an investigation into the Russian's acquisition of Portuguese citizenship.

Rabbi Daniel Litvak was arrested late last week as he attempted to leave Porto for Israel. Mr Litvak has previously defended the assessment of Abramovich's genealogy despite it being incredibly uncommon for Russian Jews to have Sephardi backgrounds.

Portuguese police said on Friday that the investigation was focusing on allegations of money laundering, corruption, fraud and falsification of documents in the process of granting citizenship to descendants of Sephardic Jews.

The Jewish community in Porto has refuted the allegations of fraud and said that they had been unfairly smeared.

The investigation into Mr Abramovich's citizenship was launched in January of this year and at the time, a spokesperson for the Chelsea owner said that they "welcome any review as it will only demonstrate the citizenship was obtained in accordance with the rules".

The oligarch is believed to have taken advantage of a Portuguese law that gives naturalisation rights to descendants of Jews that were hounded out of both Spain and Portugal during the Inquisition of the late 1400s.

In a statement to Reuters, the Justice ministry maintained that it was a routine procedure, saying: "It is a normal procedure adopted whenever there are situations or news that refer to any possible irregularity in the procedure carried out."