David Cameron is facing calls for an urgent rethink of government legislation aimed at preventing future lobbying scandals.
The chairman of the Commons committee which has been examining the Lobbying Bill has warned the plans lack credibility, dismissing them as a "dog's breakfast".
Labour MP Graham Allen, who heads the Political and Constitutional Reform Committee, said the bill would not open up the £2 billion lobbying industry to effective scrutiny.
"The new lobbying law is rushed and ridiculous," he told The Independent.
"Instead of addressing the Prime Minister's promise to 'shine the light of transparency' on lobbying, this flawed legislation will mean we'll all be back in a year facing another scandal. It is a dog's breakfast."
Mr Allen has taken the unusual step of recalling his committee during MPs' summer break for a series of special hearings to take further evidence from leading figures involved in the industry.
The bill is due to start its passage through Parliament when MPs return to Westminster, but Mr Allen is hoping the move will force ministers to rethink their plans.
It reflects concern that while the legislation would establish a statutory register of lobbyists, only firms which say it is their main business would need to register - excluding many key players.
Unions also called for urgent talks with ministers over concerns that the legislation could bar it from holding an annual conference or organising a national demonstration in the run-up to a general election.
TUC general secretary Frances O'Grady described stricter controls on spending by bodies other than political parties as "an outrageous attack on freedom of speech worthy of an authoritarian dictatorship".
A new lower limit of £390,000 in the 12 months before a national poll - now to include staff time and office costs as well as materials - is included in the Bill.
And the TUC said a new looser definition of "campaigning" risked encompassing all activities that could be seen as critical of the government of the day.
Ms O'Grady said: "It's an open secret at Westminster that this rushed Bill has nothing to do with cleaning up lobbying or getting big money out of politics. Instead it is a crude and politically partisan attack on trade unions, particularly those who affiliate to the Labour Party.
"But it has been drawn so widely that its chilling effect will be to shut down dissent for the year before an election. No organisation that criticises a government policy will be able to overdraw their limited ration of dissent without fearing a visit from the police.
"Of course not everyone agrees with TUC views and policies, but I expect there to be wide revulsion at this attack on free speech worthy of an authoritarian dictatorship. This will not just gag unions, but any group or organisation that disagrees with government - or opposition - policies."