ROME (Reuter) - Italy's battle against organised crime was in disarray last night as magistrates tried to discover whether a senior police officer arrested on Christmas Eve was a spy for the Mafia.

Bruno Contrada, an official in the civilian secret service, Sisde, was arrested in Palermo after four former mafiosi accused him of longstanding links with the organisation.

Mr Contrada, who was being interrogated by magistrates in a Rome military jail yesterday, is the most senior official to have been arrested on the evidence of Mafia informers.

His arrest shook Italy's police establishment because he spent much of his career working for anti-Mafia organisations and helped to investigate some of Palermo's most notorious murders. Before joining the secret service, Mr Contrada was an aide to the national anti-Mafia chief.

Italy's Interior Minister, Nicola Mancino, said that the charges, if true, were 'blood curdling'. According to one of the informers, Mr Contrada was a friend of leading members of at least one Sicilian Mafia family. Another informer has accused Mr Contrada of helping Toto Riina - the 'Boss of All Bosses' and Italy's most wanted man - to evade capture by warning him about a police operation in 1982.

The national police chief, Vincenzo Parisi, and other police officials were quick to defend Mr Contrada, saying that the informers may be trying to frame their former enemy. But La Repubblica, Italy's largest circulation newspaper, attacked Mr Parisi for publicly casting doubt on the work of the anti-Mafia investigating magistrates. 'If the state argues among itself, the Mafia wins,' the newspaper said.

Mr Contrada's arrest has reopened a debate on how much informers should be trusted. Magistrates have pinned their hopes on the revelations of Mafia turncoats, hoping they can help the state to defeat organised crime in the same way that informers helped to crush the Red Brigades and other terrorist groups in the 1980s.