The Tories are preparing to end the automatic Labour levy on union members if they get into power, it has been reported.
According to The Daily Telegraph, David Cameron is ready to change the law so that unions cannot simply direct millions of pounds in political contributions to the Labour Party.
Such a move would hit Labour, which is already much worse off than the cash-rich Conservatives and was heavily reliant on union funding in recent years.
The Tory leader previously called for a change in the law so that union members are asked which party, if any, they would like to make a contribution to via their dues.
The Telegraph quoted a senior party source saying the issue was still being looked at.
The claim came as shadow foreign secretary William Hague, Mr Cameron's deputy in all but name, said voters faced a choice in the forthcoming election between "change or ruin".
"If the Labour Government were re-elected they themselves would be horrified to find themselves sitting in the middle of the terrible stinking mess they have created," he told the Telegraph.
He added that it would be "too late" for voters to have a change of heart after polling day.
"There is no other chance. This country is hurtling towards massive debt and ever higher taxes under Gordon Brown.
"It is hurtling towards a position in the world that is dramatically more minor than the position that Mrs Thatcher presided over and Tony Blair was happy to exploit. It will be too late to come back in five years time."
