OTTAWA  — Prime Minister Stephen Harper’s office has issued a stern warning to Conservative senators who helped gut a controversial bill that would force unions to publicly disclose details of their spending.

On Wednesday, 16 Tory senators bucked the government’s wishes and joined their Liberal counterparts in approving a number of amendments to bill C-377, while another four abstained.

The vote represented the culmination of weeks of contentious debate in the upper chamber during which a number of Conservatives had expressed unease about the legislation, arguing it went too far and was badly written.

The amendments will now go back to the House of Commons in the fall where the government can either accept them, ignore them and send the same legislation back to the Senate a second time, or drop the matter altogether.