DAVID Cameron and Nick Clegg have been accused of cynically using the lobbying scandal to usher in trade union reforms after announcing legislation to set up a register of lobbyists will also be used to crack down on union funding of the Labour Party.

In light of the latest lobbying row at Westminster, the Coalition last night announced a Bill would be produced before mid-July to create a lobbyists' register.

However, in a surprise move, the legislation will also include measures to end the self-certification of trade union membership and reform third-party contributions to election campaigns, which would mainly affect the unions and restrict Labour's election spending power.

Asked if anyone had been consulted about the plans, the Prime Minister's spokesman said: "There has been a process of deliberation within Government on the entire package," which would "enhance the transparency of the role of third parties in the political system".

After the most recent controversy – which has seen a Tory MP and an Ulster Unionist peer resigning their party whip and two Labour peers suspended following allegations they breached parliamentary rules – the Deputy Prime Minister yesterday declared the Coalition would bring forward measures to ensure "cleaner, better politics".

The proposals will mean:

l Any body paid to lobby on behalf of a third party will be required to put its name and details of its client list on a statutory register – refusal to do so would result in financial penalties;

l Unions will be required to carry out an annual audit of their memberships, proving the figures, key in any strike ballot, are accurate;

l A change to third-party election campaign funding to ensure the true value of activities such as leaflet-printing is reflected when judging whether parties have breached the £19 million cap on campaign spending in the year before a general election.

The last proposal applies to bodies affiliated to political parties and those giving above £100,000 a year, mainly trade unions.

When calculating campaign spending they must declare, bodies such as unions would have to include not only the cost of printing a leaflet but also overheads such as staffing and rent on premises – which could potentially inflate the figures considerably.

Tom Brake, the Liberal Democrat deputy leader of the Commons, explained: "What will change is that when ministers are lobbied by companies that specialise in lobbying, people will be able to see who those companies are representing.

"When you combine that with more detailed ministerial reporting - people will be able to see why people are meeting certain organisations and what the purpose of those meetings is. That's good for transparency."

When asked about the measures relating to unions and third-party funding, he said these may have been announced prematurely.

However, a Labour source accused Mr Cameron of a shabby and panicked response aimed at diverting attention from damaging headlines hitting the Tories.

Frances O'Grady, for the TUC, said: "The Government is cynically trying to exploit a political sleaze scandal to crack down on unions, which are democratic and accountable organisations."

She said the Bill "smacks of naked opportunism, making it much harder for working people to take strike action and for unions to support local candidates; this is not the way to clean up politics".

GMB general secretary Paul Kenny said: "This is the height of hypocrisy when MPs and peers get exposed for taking money from business, the Tories and Liberals seek to deflect attention by attacking trade unions and working people."

A Unite spokesman said: "This is an outrageous attempt to point the finger of blame elsewhere by a Tory Party that accepts more than 50% of its funding from a handful of super-rich businessmen and has just had one of its MPs caught selling access to Parliament."

Meanwhile, in a further development the Speaker, John Bercow, suspended 80 passes to the House of Commons pending investigations into All-Party Parliamentary Groups by the Commons standards committee.