RAI state television and the Italian news agency ANSA said Mr Bossi confirmed that he has been formally put under investigation for suspected violation of party financing laws.
Mr Bossi spent more than two hours with Prosecutor Antonio Di Pietro, the Milan magistrate who began a series of corruption investigations into Italy's business and political establishment almost two years ago.
The involvement of the Northern League in the investigation - the last major political force to have been almost completely untouched by the scandal - comes days before Italy's president is expected to dissolve parliament and set elections for the spring to replace the largely discredited legislature.
Unlike the long-dominant Christian Democrat and Socialist parties, which have been disgraced by kickback investigations, Mr Bossi's League came under the scrutiny of the prosecutors for alleged violation of party funding laws.
'I spontaneously gave (Di Pietro) the ledger sheets' to examine, Mr Bossi said, adding that the investigation involves 'irregularities and not kickbacks'.
In recent local elections Mr Bossi's party rocketed to power in many northern cities, including Milan, Italy's financial capital.
Mr Bossi told reporters that as leader of the Northern League he was assuming all responsibility in connection with an undeclared 200 million lire (pounds 95,000) contribution to the party's coffers by the Italian chemical giant Montedison. Alessandro Patelli, an administrative official of the Northern League, was arrested this month in connection with investigations into the contribution.
Northern League candidates have campaigned on the boast that the party is innocent of the corruption which was a way of life for the established parties and have called for more autonomy from Rome's central government.
Investigators have been examining Montedison's books after allegations from politicians that the company made millions of dollars of payoffs to politicians in connection with its operations.Reuse content