Not less than ninety days nor more than one year after the first meeting of the metropolitan charter commission, a proposed district charter shall be submitted to the electors of each town, city or borough to be included in such metropolitan district at the next general election unless a petition for a special election is filed with the clerk of two or more towns, cities or boroughs to be included in the proposed district signed by not less than ten per cent of the electors of such towns, cities or boroughs. The sufficiency of such petitions shall be determined in the same manner as is specified in section 7-336. If the charter commission, by unanimous vote, determines that more than one year is required for the completion of its duties, an extension of time, not to exceed ninety days, shall be permitted. The metropolitan district charter shall become effective if approved by a majority of the electors voting thereon in accordance with the provisions of this section; provided such majority shall be no less than fifteen per cent of the total number of electors in all the towns, cities and boroughs voting on the question.