Formally, she is charged with dereliction of duty and revealing details of a case. If found guilty, she would be the first judge condemned on such charges in Spain's modern history. That could mean two months in jail, suspension for three years and a fine of 100 million pesetas (nearly pounds 500,000 ). Her Marbella home has already been 'embargoed' - held as collateral - to cover the possible costs of the trial.
Effectively, someone is trying to run her out of town. When Judge Diez, from the northern Asturias region, then only 27, hit Marbella four years ago, it was as though a young female Wyatt Earp had blown in to clean up the town. Thousands of Marbellans have marched in her support and five leading citizens are approaching one month on hunger strike, demanding an end to judicial corruption.
'The Spanish Inquisition continues, quite literally,' said one of the hunger strikers, a retired businessman, Fernando Rosado, 62. 'Justice in Spain is the best money can buy. There must be some honourable judges but they are cowards. By being cowardly, they're protecting the corrupt.'
For once, when the British tabloid press refer to the Costa del Sol with their favoured Costa del Crime headlines, they are not wrong. British, Italian, Arab and other high- flying criminals, live and operate here in style, flaunting their wealth with large yachts at the nearby Puerto Banus marina. Judge Diez believes she has found out some of the reasons why.
She compiled dossiers that she said linked former judicial officials here with the Sicilian Santapaola Mafia clan. Her dossiers suggested illegal laundering of Italian Mafia money through the purchase of art works, antiques and, especially, property.
Based on investigation of past cases and judgements, as well as sworn statements from witnesses who wrote to her supporting her anti-corruption stand, Judge Diez's dossier centred on one of Marbella's most influential figures, a former judicial official called Juan Ramirez. Mr Ramirez's exact profession has never been clear, but he worked in the local courts and often acted as a legal adviser to accused persons.
Mr Ramirez, in his sixties and known to some business associates as 'el flaco' (Skinny), is also the father of one of Ms Diez's fellow Marbella judges, Pilar Ramirez, and of a local lawyer, Juan Carlos Ramirez. Judge Diez's lawyer, Luis Bertelli, contends that, under Spanish law, such family ties are illegal in a town of fewer than 10 judges. Marbella has seven judges, including Ms Diez. Mr Bertelli also says Mr Ramirez has acted as a lawyer illicitly since, according to Mr Bertelli, he has no qualifications.
Mr Ramirez has long been a friend of Carmen Proetta, 50, a resident of San Pedro de Alcantara, near Marbella, and the key witness in the 1988 SAS killing of three IRA terrorists. Ms Proetta has said she acted as an interpreter for Mr Ramirez in cases involving Britons.
Acting on witnesses' allegations, Judge Diez ordered Mr Ramirez's phone legally 'bugged' in January last year. She later ordered him jailed in 'preventive detention' pending trial, on suspicion of involvement in the fraudulent sale of a Marbella casino.
After two months inside, another judge - not his daughter - ordered Mr Ramirez freed without bail. Mr Ramirez made a formal complaint against Judge Diez, a move that automatically led to her suspension and tomorrow's trial. He accused her of dereliction of duty and of revealing details of his case. She denies both accusations, saying Mr Ramirez and corrupt colleagues are trying to get her off their patch.
According to Mr Bertelli, the phone tap revealed a close friendship between Mr Ramirez and Jose-Luis Manzanares, vice-president of Spain's General Council of Judicial Power, the legal watchdog body appointed by parliament.
That relationship has turned the judge's trial into an event of national importance.