Italian Prime Minister Romano Prodi says he has instructed his lawyers to take legal action over what he called damaging allegations made against him.
A senator demanded checks into Romano Prodi's background
It follows the publication of reports that under the last government, an academic was asked to investigate if Mr Prodi had any ties with the KGB.
The request is said to have been in the context of an official inquiry into KGB activities in Italy.
There is no evidence of any links between Mr Prodi and the KGB.
Published transcripts of phone conversations suggest that in the run-up to the general election in April, the chairman of the commission looking into KGB infiltration in Italy, Paolo Guzzanti, asked an academic, Mario Scaramella, to find out if Mr Prodi had any KGB links.
Mr Guzzanti, who is a senator for the centre-right Forza Italia party, which led the previous government, said he had been quite open about asking for Mr Prodi to be vetted.
Mr Scaramella was confirmed on Friday to have been contaminated with polonium-210, the same substance that apparently killed KGB defector Alexander Litvinenko in London.
Mr Scaramella had met Mr Litvinenko at a sushi bar in London on 1 November, the same day that the latter was taken ill.
The Italian academic said he had met Mr Litvinenko in the course of his investigation into KGB infiltration.