The former Archbishop of Canterbury, Lord Williams, has urged the Government to make fundamental changes to the Lobbying Bill out of concern that it could hamper the freedom of charities to campaign in the run-up to elections.

The Bill, which seeks to rein in the influence of corporate lobbyists, has proved hugely unpopular.

The Trades Union Congress has referred to the Bill as "an outrageous attack on freedom of speech" while Political and Constitutional Reform Committee chairman Graham Allen has called it a "dog's breakfast" and "an absolute disgrace".

The Commons made several amendments to the legislation last week but MPs voted against amendments to the regulation of staff costs and spending limits within parliamentary constituencies, despite these changes having passed by a majority in the House of Lords.

The Bill returns to the House of Lords today and will be debated in the House of Commons on Wednesday.

Over 130 civil society organisations, including Christian Aid - of which Dr Williams is the Chairman - Oxfam, the Royal British Legion, Amnesty International and the National Federation of Women's Institutes, opposed the original Bill and welcomed the changes made in Parliament last week.

However, the general consensus is that enough has not yet been done to protect charities under the new legislation, which aims to limit election campaign spending by groups not actually standing for election.

In a letter to Cabinet Officer Minister Chloe Smith, charities have complained that: "The complexity of the legislation, the lack of clarity in drafting, the amount of discretion given to the Electoral Commission in determining how the rules apply, and the remarkably burdensome reporting requirements... will collectively have the result of muting charities and groups of all sorts and sizes on the issues that matter most to them and the people they support."

A spokesperson for the Cabinet Office has assured that the Bill will ensure transparency and accountability within the political system and that organisations without an electoral agenda will not be affected by the law, but concerns have been raised that unclear wording could mean charities such as Christian Aid are affected, despite having no political sway.

"There is remarkably widespread resistance from charities and voluntary organisations to some aspects of the Lobbying Bill currently going through Parliament, and this should give the Government reason to pause," said Lord Williams, who has branded the legislation as "gravely inadequate".

"The problem identified in the parliamentary debates is that a Bill designed to increase transparency in our democracy, and to curtail unaccountable and potentially corrupt influence, could have the unintended effect of burdening and weakening these civil society agencies in a way that is seriously bad news for democracy," he said.

"We hope that the Government will think again about the potential danger of this legislation for the freedom of civil society groups to participate in public debate in the run-up to elections.

"The debate is not over," he finished.