Downing Street has rejected warnings that new legislation on lobbying will prevent charities from speaking out on matters of public interest.
Only the small number of charities which explicitly promote particular parties or candidates will be caught by the provisions of the Transparency of Lobbying, non-Party Campaigning, and Trade Union Administration Bill, said Prime Minister David Cameron's official spokesman.
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, while official democracy watchdog the Electoral Commission has warned of concerns about the "workability" of the legislation.
An electoral law expert commissioned by the National Council for Voluntary Organisations (NCVO) to assess the likely impact of the reforms - due to be debated in the House of Commons tomorrow - concluded that they could have a "chilling effect" on charities.
" Uncertainty about what the law requires is likely to have a chilling effect on freedom of expression, putting small organisations and their trustees and directors in fear of criminal penalty if they speak out on matters of public interest and concern," wrote Helen Mountfield QC in a legal opinion that has been submitted to the Cabinet Office.
"The restrictions and restraints are so wide and so burdensome as arguably to amount to a disproportionate restraint on freedom of expression."
But the PM's spokesman told a Westminster media briefing: "Of course we 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.
"My understanding is that at the 2010 general election, very few charities were registered as third parties. That is why we are saying that, provided charities continue to campaign as the vast majority of them always have they won't be affected."
The Government's proposals would introduce a statutory register of lobbyists to identify whose interests were being represented by consultant lobbyists and those who were paid to lobby on behalf of a third party, and set a £390,000 cap on the amount any organisation - excluding political parties - could spend across the UK during elections.
Last week the Electoral Commission, the independent body responsible for overseeing the UK's electoral system, raised a series of concerns about the bill warning of some ''significant issues of workability''.
Karl Wilding, director of public policy at NCVO, said: " This bill takes us from a situation in which charities and community groups largely understood the rules on what they could do, into a position where no one has any idea what the rules are, but may nevertheless face criminal prosecution for getting them wrong.
" This is the inevitable consequence of rushing legislation through without any consultation.
" I would like the Government to give serious consideration to putting its proposals on hold. This would give them the chance to consult properly on a solution that addresses concerns about undue influence in politics without the risk of sweeping every charity and community group in the country into a deeply burdensome bureaucratic regime."
Commons leader Andrew Lansley and constitutional reform minister Chloe Smith will meet representatives from the NCVO at the Cabinet Office in an attempt to address their concerns.
Ms Smith said: " My fellow ministers and I are very happy to listen to the concerns and ideas of organisations affected by this legislation, but I hope to be able to put the NCVO's minds to rest that we do not intend to capture a huge swathe of their membership who are not already registered as third party campaigners.
"At the 2010 general election, very few charities were registered as third parties. Provided they continue to campaign as most of them always have - that is, they are not promoting the electoral success or otherwise enhancing the standing of parties/candidates - charities will not be affected by this legislation.
"However, we have also been clear that we want to take unaccountable money out of politics. So the Bill does seek to regulate the spending of organisations seeking to promote a particular outcome of an election."