LISBON (Reuters) - Portugal's supreme court rejected on Wednesday a request for former prime minister Jose Socrates to be freed from prison after his arrest for suspected tax fraud and corruption, offences he has denied committing.He is the first Portuguese ex-premier to be detained since Portugal became a democracy 40 years ago, as prosecutors and judges intensify a campaign against corruption in a country notorious for its snail-paced justice system.The supreme court said it had spurned the request on Socrates's behalf, filed by a private individual, because of a "manifest lack of legal foundation" for the release.It ruled that the "preventive custody of Jose Socrates does not show an illegitimate challenge to his individual liberty".Socrates' lawyer has said he is appealing against the arrest.Socrates, taken into custody at Lisbon airport last month, has said the accusations against him are groundless and his detention amounts to "gratuitous humiliation."It is not clear whether the investigation, named "Operation Marquis", covers Socrates' term as premier from 2005 to 2011.During that tenure, he weathered several investigations, touching in part on allegations that he misused his post as environment minister in 2002 to allow the construction of a shopping mall. He denied wrongdoing and faced no formal charges.A series of high-profile legal cases has rocked the economically troubled European Union country this year. They include the detention of prominent civil servants over the granting visas to wealthy foreigners, and an inquiry into the collapse of the Espirito Santo banking family's business empire.(Reporting By Axel Bugge)Join the conversation about this story Click here to read full news..