Ratings agency Standard & Poors hit back at Italian prosecutors on Thursday after sources said its offices in Milan were visited by tax police pursuing a probe into the impact of S&P's reports on Italian share prices.

S&P is surprised and dismayed by these investigations into our independent ratings, S&P said in a statement sent by email. The claims being made are baseless and entirely without merit, and we will vigorously defend our actions, our reputation and that of our analysts.

A source close to S&P confirmed Italian tax police had carried out the visit but declined further comment.

An investigative source said the move was part of a probe being carried out by prosecutors in the southern Italian town of Trani.

I only know that it's part of the Trani investigations, but I don't know any more, S&P defence lawyer Giuseppe Fornari said on his way in to the S&P offices.

In August Italian prosecutors seized documents at S&P's offices in Milan following complaints from Italian consumer groups over the impact of its reports about Italy on Milan stock prices.

One of the complaints filed in May last year targeted Standard & Poor's after it threatened to downgrade Italy's credit rating because of its huge public debt.

Prosecutors are investigating whether crimes of market manipulation and illicit use of privileged information were committed when S&P's reports were released in May, June and July 2011, prompting a sell-off of Italian assets.

Earlier this month S&P hit the euro zone with a downgrade of half the countries in the single currency area, including Italy, fuelling long-standing frustration with rating agencies.

A day after the downgrades, a leading political ally of German Chancellor Angela Merkel urged legislation to reduce the reliance of institutional investors on ratings agencies.

Separately on Thursday, sources said tax police had carried out routine checks at S&P's Milan headquarters at the end of November, which had thrown up interesting findings, but added that Milan prosecutors had not opened an investigation.

(Additional reporting by Stephen Jewkes; Editing by Hans-Juergen Peters and David Holmes)