* No bail for central figure in major corruption case
* Judge says he failed to explain origin of 48 mln euros
* Barcenas denies charges against him
* His lawyer says will file appeal to get him out of jail (Recasts, adds reaction, defence lawyer, detail, background)
By Teresa Larraz Mora
MADRID, June 27 (Reuters) - A former treasurer for Spain's ruling People's Party was sent to jail without bail on Thursday as the High Court continues a pre-trial investigation into corruption charges against him.
Judge Pablo Ruz said that Luis Barcenas - who worked for the People's Party for almost three decades, mostly in the accounting department - had failed to explain the origin of up to 48 million euros held in Swiss bank accounts.
Ruz said Barcenas was a flight risk and could also pressure witnesses or destroy evidence.
Barcenas is the central figure in two major corruption scandals that have damaged the credibility of the ruling party and angered Spaniards who are struggling with a deep recession and high unemployment.
The 55-year-old has repeatedly said he is innocent of the charges against him and that his money came from legitimate business activities.
His lawyer Alfonso Trallero said he would file an appeal to try to get his client out of jail, El Pais newspaper reported.
Barcenas is charged with money laundering, bribery, tax fraud and other crimes in Ruz's investigation of a group of businessmen who put on campaign rallies and other events for the People's Party in exchange for kickbacks.
Ruz wrote in his 24-page order that he had unearthed up to 48 million euros in bank accounts linked to Barcenas in Switzerland and other countries.
The judge also said in the order that Barcenas had falsified documents and invented art deals to try to hide the source of the money.
Barcenas appeared before Ruz on Thursday in a closed-door hearing to answer questions regarding new accounts that were discovered in Switzerland and about the art deals.
Barcenas' explanations have been "partial, inconsistent and manifestly insufficient regarding the activities and movement of assets in his Swiss accounts, which he himself has been involved with", the judge wrote in his order.
Barcenas has been under investigation off and on for more than four years over the kickbacks inquiry, which Ruz said was approaching the trial phase. As an examining magistrate, Ruz both investigates the charges and presides at trial.
Several of the charges against Barcenas carry potential prison terms of six years each, and large fines. The anti-corruption prosecutor has asked the judge to order Barcenas to put up 28 million euros in guarantees to cover potential fines should he be convicted.
In a separate case Barcenas is also under investigation for allegedly running a People's Party slush fund that took donations from construction magnates and redistributed them in cash payments to party leaders.
In January, El Pais published excerpts of two decades of handwritten accounts - now known as the 'Barcenas Papers' - allegedly recording movements of cash in and out of the secret fund.
Prime Minister Mariano Rajoy, former Prime Minister Jose Maria Aznar and other top People's Party officials whose names appear in the Barcenas Papers have repeatedly denied receiving illegal payments.
Some party leaders have said the party paid them fixed expenses that were legal and reported on their tax declarations.
"As always the People's Party manifests its respect for judicial decisions in all proceedings," the party said in a one-line statement on Thursday.
Barcenas left the party in 2009, when he was first put under criminal investigation, but continued to receive severance pay on an instalment plan until early this year. (Writing by Fiona Ortiz; Editing by Julien Toyer and Pravin Char)