Lionel Messi could face a prison sentence and a huge fine if he is convicted of tax fraud, a lawyer has said.The Barcelona striker and his father are under investigation in Spain for allegedly defrauding the state of more than four million euros.Officials claim that Messi and his father, Jorge Horacio, filed fraudulent tax returns for 2007 to 2009 - a crime that could see them jailed for more than two-and-a-half years and fined up to 24 million euros if they are found guilty.Messi denied the claims in a statement on his Facebook page on Wednesday, with the Argentine insisting that "we have always fulfilled all our tax obligations".