More than 20,000 civil servants have joined a pro-Beijing umbrella union rather than its pro-democracy rival since the handover.
The number of government staff represented by the Government Employees' Association, a long-established civil service union belonging to the veteran pro-Beijing Federation of Trade Unions, has increased from 15,000 to 35,000 over the past two years.
Ten unions - some of which are also members of the Government Employees' Association - have joined the federation directly. The 10 represent 2,600 staff.
They include unions representing employees from the municipal services, which will be restructured after the provisional municipal councils are scrapped.
The federation represents 280,000 employees.
Federation chairman Cheng Yiu-tong says government trade unions may have joined his group because they believe it has more bargaining power with the Government due to a good relationship with the administration.