The coalition faces a lengthy standoff over charities' ability to campaign against government policies when parliament returns next week.
Shadow leader of the House Angela Eagle used a speech in east London to attack the government's lobbying bill, which ministers say is aimed at taking third-party funding out of politics.
The opposition fears charities could be forced to stop campaigning in coalitions, pull back from public-facing activity to avoid hitting a lowered spending limit and reduce their lobbying efforts because of the increased administrative burden.
The transparency of lobbying, non-party campaigning and trade union administration bill has already been criticised by the National Council for Voluntary Organisations, which has warned the measures would have a "huge impact on charities' and other groups' day-to-day work".
Now Eagle has thrown the opposition's weight behind their concerns by pledging to fight the government on the issue when the bill comes to the floor of the Commons. It is the only major piece of government legislation to face scrutiny in the two-week session before this autumn's conference season.
"The big money in politics today is spent by political parties not by charities and campaigning groups," she said.
"Yet this bill looks to gag the latter, while doing nothing to curtail the former which spent ten times more than all third parties put together in the runup to the last general election."
The proposed new rules are being drawn so widely they could prevent a health charity from publishing a leaflet warning of the dangers of smoking, it is feared.
A local community group opposing a bypass could become subject to regulation if a member subsequently went on to stand in a local election, the NCVO has warned.
"It is deeply worrying that large numbers of organisations would be unable to know with any degree of certainty whether they were acting within the law, leaving them open to the risk of prosecution," a letter from the NCVO's chief executive Sir Stuart Etherington said.
Eagle called on Cameron to rewrite the bill so that it protects the big society and "properly regulates the lobbying industry".
Under present plans only lobbying firms working for external clients will have to sign up to the statutory register of lobbyists. In-house lobbyists, which make up the majority of the public affairs industry, would not be affected.
"This bill is a cynical attempt by the government to insulate their record and their policies from legitimate, democratic criticism in an election year," she added.