Senior Tories are seeking to ambush Ed Miliband by tabling legislation to force unions to end automatic affiliation fees paid to Labour.
They are in talks with Liberal Democrats over introducing the change in an anti-sleaze bill, which would also include a register of lobbyists.
“We want to put Ed Miliband on the spot,” said a senior Conservative.
The move would heap pressure on the Labour leader to press ahead swiftly with switching to a system of union members having to opt in to paying a levy, of around £3, to Labour, rather than being offered the chance to opt out.
Mr Miliband proposed the change in the wake of claims that the Unite union sought to rig the selection of a parliamentary candidate in Falkirk, allegations it denies.
Deputy Prime Minister Nick Clegg has accused Mr Miliband of “making a great song and dance” about changing the affiliation fee rules.
Labour insiders estimate the reform could cost the party £5 million in union funding.
