Unite has called the Prime Minister's decision to include curbs on trade union election spending in the lobbying bill "a grotesque insult".
A spokesman for the trade union said: "This is an outrageous attempt to point the finger of blame elsewhere by a Tory Party that accepts more than 50 percent of its funding from a handful of super-rich businessmen".
"Union funding for the Labour Party comes from individual members, ordinary men and women, and is totally transparent.
"How dare this Government suggest that their membership is somehow on the par with a man who defiled the office of MP by hawking himself for a few quid. This is a grotesque insult".
The Government will bring forward a bill to introduce a statutory register of lobbyists before Parliament's summer recess in July.
The hastily announced Government bill to create a statutory register of lobbyists is a highly politicised piece of legislation.