The government should meet the Electoral Commission, the Charity Commission and the National Council for Voluntary Organisations in order to finalise the wording of the lobbying bill, according to Sir Stuart Etherington, chief executive of the NCVO.
The government has promised to introduce an amendment to protect charities from changes introduced in the Transparency of Lobbying, Non-Party Campaigning and Trade Union Administration Bill.
The bill contains proposals to make it a criminal offence to spend more than £390,000 on campaigns that affect European, national and local elections in the year before those elections.
Charities have warned that the definition of election campaigning is so wide that it could catch many legitimate charitable activities, and elections are so frequent that the new law would, in effect, stop many charities campaigning altogether.
In a letter to Andrew Lansley, the leader of the House of Commons, Etherington said that charities "should be secure in the certainty" that they can campaign for a change in the law.
"Charities and voluntary organisations that are simply active in trying to change policy for the advancement of their own mission should have the guarantee that they can continue to campaign as vigorously as they wish in putting forward their policies," said Etherington.
"Government has on many occasions stated that its intent is for the new rules to not affect organisations campaigning on policy issues. We are expecting to see an amendment that is drafted with wording to that effect."
Etherington wrote that once a draft amendment was available, he would strongly urge Lansley and his officials to arrange a round-table meeting involving the Electoral Commission, the Charity Commission and a representative group of organisations, including the NCVO. "This would provide all interested parties with an opportunity to discuss any outstanding concerns about the new wording," he said.
Etherington also outlined a number of remaining problems with the bill. He said the list of activities that counted as lobbying was "vague and excessively burdensome", that proposed limits on expenditure were too low and that there were no practical proposals to regulate groups working in coalition.
The government said during the committee stage of the bill that it would be amended before the report stage of the bill, which is scheduled for 8 October.