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The Vatican has said it is troubled by the latest sex allegations made against the Italian prime minister.

Magistrates on Friday opened an investigation into Silvio Berlusconi, alleging that he had paid an under-age prostitute.

He has dismissed the investigation as politically motivated and vowed to punish the magistrates behind it.

Cardinal Tarcisio Bertone said those in authority should show a more "robust morality".

"The Church urges and invites everyone, especially those who hold a position of public responsibility [...] to commit themselves to a more robust morality, a sense of justice and legality," Cardinal Bertone said, in a rare public and openly critical comment by the Holy See on the matter.

'Unacceptable charges'

He said the Vatican, which was following the matter "with great attention and concern", shared the concerns of the Italian President Giorgio Napolitano.

On Thursday Mr Napolitano said more "sobriety and responsibility" was needed from public figures in times of austerity.

The cardinal's comments came a day after the prime minister broadcast a 10-minute TV message, denouncing the investigation as procedurally flawed and vowing to pass new laws to prevent magistrates pursuing elected officials.

The president of the Italian magistrates' association, Luca Palamara, told Italy's SkyTG24 television network Mr Berlusconi's comments were "unacceptable" and "seriously threatened the autonomy and independence of the prosecutors".

Much of the investigation focuses on Karima El Mahroug, an 18-year-old Moroccan belly-dancer who attended Mr Berlusconi's parties when she was 17 and, prosecutors say, was paid to have sex with him. Sex with a prostitute aged under 18 is an offence in Italy.

Both Mr Berlusconi and Ms Mahroug have denied having sexual relations, and she has described a sum of 7,000 euros (£5,900) that he gave her as a gift.