The Government has come under pressure to put its lobbying reforms on hold after two separate parliamentary reports today raised fears that the changes will curb free speech.
Legislation to impose tighter controls on the industry are being rushed through parliament without enough time to be properly scrutinised, MPs and peers claimed.
The new laws are unlikely to increase transparency or restore public confidence in the political system, according to the House of Lords Constitution Committee.
Meanwhile, the Joint Committee on Human Rights called for a "pause" in the legislation to allow major concerns to be tackled. It said that, while there may be a "pressing need" to reform non-party campaigning, parliament had not been given enough time to assess if the changes interfering with free speech were justified.
The Transparency of Lobbying, Non-Party Campaigning and Trades Union Administration Bill introduces a register of lobbyists and their clients and imposes a limit on the amount organisations other than political parties or candidates can spend on campaigning during parliamentary elections.
Hywel Francis, chairman of the JCHR, said: "My committee accepts that measures to protect the electoral process are, in principle, a clear legitimate aim. We welcome the amendments brought forward by the Government at Commons report stage, insofar as they improve the clarity of the Bill. However, we remain concerned about a number of issues raised by this Bill.
"My committee also finds it unacceptable that it has not been able to report on a Bill that raises significant human rights issues before it has left the first House, on account of the unnecessary speed at which the Bill is being taken. This amounts to an abuse of the Parliamentary legislative and scrutiny process - and this is not the first time that this has happened during this parliament.
"We call on the Government to 'pause' this Bill to allow for further scrutiny and consultation, particularly with the Electoral Commission, the Commission on Civil Society and Democratic Engagement and other stakeholders.
"If this does not happen, we recommend that the Bill should be amended to remove both the lower thresholds for registration and reduced spending limits, and leave them at their current level pending further detailed work on whether the current limits are appropriate."
Under the reforms, a statutory register of lobbyists would 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. Trade unions would be required to provide accurate membership lists.
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.
Ministers were forced to later make changes to the proposals clarifying rules on third-party political spending to make clear that public rallies rather than member-only meetings are to be regulated and spell out that campaigners who respond to policy questions by the media are not captured by the Bill.
But charities still insist the changes will impact on their activities by limiting their spending on campaigning activities in election years.
Baroness Jay, chairman of the House of Lords Constitution Committee, said: "The committee is concerned about the restrictions on the right to freedom of political expression that will result from the proposal to limit third-party expenditure at general elections. We think this constitutional right should only be interfered with where there is clear justification for doing so.
"We are also concerned that the lobbying Bill will not achieve its objectives of increasing transparency and restoring public confidence. We have therefore recommended that the House of Lords considers whether the limited definition of lobbying in the Bill, which excludes in-house public affairs work and covers only communication with ministers and permanent secretaries, will provide adequate transparency.
"We are critical of the hurried way in which this legislation has proceeded, which has resulted in a lack of consultation. Bills of constitutional importance such as this should not be rushed through parliament."
TUC general secretary Frances O'Grady said: "This is the second all-party committee of Parliament to give the lobbying Bill a good kicking.
"The continuing refusal of ministers to withdraw the Bill and start again with a proper process of consultation and a genuine aim to seek the widest possible agreement reveals the shabby partisan motives at its heart.
"The Government has not even been able to say what exactly the problem is the Bill is meant to deal with. The only conclusion is that they want to use the law to hobble people who disagree with them."