Supporters of a wide range of charities and campaign groups are descending on Parliament to urge MPs and peers to rewrite legislation which they fear will have a "chilling" effect on free speech at election time.
A new report from the Commission on Civil Society and Democratic Engagement has warned that the Lobbying Bill needs urgent changes to prevent "significant damage to legitimate campaigning".
The Bill returns to the House of Lords next week after ministers granted an unusual five-week pause to head off possible defeat at the hands of peers.
The Government has already tabled numerous amendments to the Transparency of Lobbying, Non-Party Campaigning and Trades Union Administration Bill, amid fears that its proposal for a cap on third-party spending on political campaigns during the 12 months before a general election would stop charities and other bodies from speaking out about issues which are central to their causes.
The cross-party Civil Society Commission, chaired by former bishop of Oxford Lord Harries, called for further changes including:
:: Treating campaigners in the same way as political parties by excluding staff costs from spending limits;
:: Reducing the period covered by the legislation to six months ahead of an election instead of a year;
:: Dropping the proposed tightening of spending caps for campaigners;
:: Doubling the current spending levels at which campaigners have to register with the Electoral Commission regulator;
:: Scrapping the proposed constituency spending limit .
The Commission warned that the Government has so far failed to make any meaningful changes to the legislation, despite acknowledging the need for a five week pause in its passage through Parliament to allow further work.
Supporters of groups from across the political spectrum are expected to lobby MPs and peers to heed the Commission's recommendations.
James Legge, of the Countryside Alliance, said: "It is unacceptable for the Government to be rushing ahead with legislation it has acknowledged is vague and potentially wide open to wrong interpretations by promising to give a quango the power to decide what the law means. This uncertainly will hang over campaigns of all sizes and will see some decide they cannot take the risk of campaigning at all."
Mark Goldring, chief executive at Oxfam, said: "From fair trade and Make Poverty History to our championing of the anti-apartheid movement, Oxfam has always known it has to campaign in the world of politics to tackle the root causes of poverty.
"But this is very different from being party-political. We welcome the Commission's sensible package of measures that seek to bring transparency to election campaigning whilst allowing Oxfam to continue to champion the interests of the world's poorest people."
And Dawn Varley, director of campaigns at the League Against Cruel Sports, said: "There are more than 100 charities and campaign groups supporting the Harries Commission. We may not always like what other organisations say, but we will defend their right to say it. It's not too late for the Government to sort out this mess."
A Cabinet Office spokesman said: " These are important reforms to bring greater transparency and accountability to the political system.
"We have provided extra time before the House of Lords debates part two of the Bill, to allow for further discussion and consultation with campaigning groups, and we will be listening carefully to the views expressed in the Lords next week.
"The Government has already indicated that we will be reconsidering the thresholds for registration with the Electoral Commission
"This report is another welcome addition to the debate, and we are pleased that the Commission has endorsed our principle that third parties who campaign at elections should be subject to regulation.
It would be premature to respond on any specific recommendations before the period of discussion has finished and before we have had the opportunity to hear the views of parliamentarians."