The government's Lobbying Bill is a "sinister gag" on democracy, the Labour Party has warned.
Reforms to lobbying rules will be debated by MPs today amid warnings they will gag charities and voluntary bodies while failing to provide the transparency needed to prevent political scandals.
Angela Eagle, the shadow leader of the Commons, said the Bill should be re-written as in its current form it was "actually a step backwards from the codes of conduct and sanctions that already exist".
Writing on The Huffington Post UK, Eagle said the legislation was "a sop to powerful vested interests and a sinister gag on democratic debate, and it shows just how out of touch David Cameron and his government are".
"This is a draconian, illiberal Bill that lets vested interests off the hook but prevents civil society from having a say. This is shocking even from the Tories, but the Liberal Democrats should be truly ashamed. In a bill that is neither liberal nor democratic, they would rather fight the Tories' corner than fight for free speech."
She added that the government should rename it the 'Let Lynton Lobby Bill' - a reference to Conservative election strategist Lynton Crosby.
Eagle said: "This bill wouldn't stop arch-lobbyist Lynton Crosby setting the government's tobacco policy, but could stop an organisation like Cancer Research UK or the British Medical Association from campaigning about it."
Charity sector chiefs emerged from talks yesterday with ministers unpersuaded by assurances about the Transparency of Lobbying, non-Party Campaigning, and Trade Union Administration Bill.
The legislation would set a £390,000 cap on the amount any organisation - excluding political parties - could spend across the UK during elections.
A statutory register of lobbyists would also be introduced to identify whose interests were being represented by consultant lobbyists and those who were paid to lobby on behalf of a third party.
The Bill was published in July following allegations about the influence of lobbyists on government decision making as well as the involvement of peers and MPs with lobbying groups.
The Government though is under mounting pressure to significantly amend the proposals - or abandon some elements altogether - from a broad spectrum of critics.
They include the Electoral Commission, the official democracy watchdog, which warned of "significant issues of workability" and uncertainty for groups that campaigned on public policy.
Oxfam, the Royal British Legion, and the Salvation Army are among key organisations that fear the coalition Bill is so complex and unclear that it is likely to be "impossible" to follow.
The TUC has claimed it could even prevent it holding an annual conference in what general secretary Frances O'Grady called "an outrageous attack on freedom of speech".
Concerns have also been raised that the register will catch only a tiny proportion of those paid to seek to influence government policy by excluding those for whom it is not their main business.
And the Commons committee which has been examining the Bill held emergency sessions last week to discuss concerns about what its chairman called "rushed and ridiculous" proposals.
On Tuesday morning the prime minister's official spokesman said the government "always listen to concerns".
"However, my understanding is that - provided the charities are not promoting the electoral success or otherwise enhancing the standing of a particular party or political candidate - they will not be affected by this legislation," the spokesman added.
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