Under FIFA rules a footballer cannot play competitively for two clubs between July 1 and June 30 the following year and Mascherano has already appeared for both Corinthians in Brazil and West Ham in England during that period.

FIFA have ruled, however, that the 22-year-old, who only completed two full matches for West Ham in a total of just seven appearances, was an exceptional case.

In a statement FIFA explained their ruling was determined partly because the South American and European seasons overlap.

"FIFA looked at this from a global perspective and the overlapping aspect of the seasons was taken into account," added a spokesman.

A statement said: "The FIFA single judge ruled that Javier Mascherano is eligible to play in official matches with Liverpool with immediate effect."

Mascherano, who played in every minute of all of Argentina's five matches in last year's World Cup finals, signed for West Ham with international team mate Carlos Tevez on the final day of the summer transfer window last August.

He last started a match for the Hammers in the 1-0 defeat at Tottenham on Oct. 22 and since then has only made one six-minute appearance as a late substitute in the 2-0 defeat at Everton on Dec. 3.

He is expected to sign an 18-month deal at Anfield with Liverpool coach Rafa Benitez telling the club's Web site: "He will give us options and is a very good character."