The Hong Kong government is facing a potentially bitter “referendum” campaign from two pro-democracy parties, in spite of securing legislative approval for a $8.6bn express rail line at the weekend and overcoming a protest movement that had delayed the project for the past two weeks.

Five legislators from the Civic party and League of Social Democrats intend to resign their seats on January 27 and contest the ensuing by-elections, which they say will constitute a de facto referendum on a faster pace of democratic reform.

The Chinese government has ruled out direct elections for Hong Kong's chief executive until 2017 at the earliest – and 2020 for the full legislature. At present, only half of the Legislative Council's 60 seats are filled by direct elections. The remainder are reserved for “functional constituencies”, representing mainly pro-Beijing business and professional sectors.