MILAN, June 1 (Reuters) - Italy's top insurer Generali is expected to launch its takeover bid for smaller rival Cattolica between late September and early October, a source close to the situation said on Tuesday, adding the buyout offer would end in November.

Generali on Monday said it would launch a 1.17 billion euro ($1.43 billion) takeover offer for Cattolica , which it expects to conclude by the end of the year, to cement its domestic market leadership.

Generali has about 20 days to prepare the documents it needs to submit to industry and market authorities to gain clearance for the offer, with a decision by regulators expected after the summer, the source said.

Among conditions set for the offer to be effective, Generali has included an unconditional approval of the deal by Italy's antitrust authority. ($1 = 0.8166 euros) (Reporting by Gianluca Semeraro; editing by Giulia Segreti)