Proposals to extend the lobbying bill, which is to be debated at Holyrood today, will reduce democratic engagement and stifle the voice of charities, a leading voluntary sector body has claimed.

The Lobbying Bill, which will be the subject of a stage one debate at the Scottish Parliament, is designed to increase transparency and introduce a registration scheme for lobbyists who directly engage with MSPs and Scottish Ministers.

However many communications with politicians will not be covered by the current plans and the Scottish Parliament's standards committee and others have called for the proposed law to be strengthened.

Now the Scottish Council for Voluntary Organisations (SCVO) has renewed warnings that charities will be affected by existing plans and any extension to the types of communications classed as lobbying.

Currently the law only calls for face-to-face meetings with politicians to be declared, but organisations including Spinwatch and the Law Society of Scotland have said lobbying by email, telephone, post and video-conferencing should also be included.

In a briefing to MSPs SCVO has warned that the bill risks "sabotaging the inclusive and participatory nature of the Scottish Parliament", and will have a chilling effect on the ability of charities to campaign.

While transparency, especially about corporate lobbying, may be desirable, it should not be at the cost of protecting participation, the charities umbrella body says. If charities, especially small ones, have to register as lobbyists every time they attend a parliamentary reception, for example, many may just stop going.

John Downie, SCVO director of public affairs, said: "We fear MSPs are sleepwalking into legislation which will reduce the likelihood of people engaging with politicians. We agree with the Scottish Government's original proposal of light touch, proportionate regulation to improve transparency, but this is heavy-handed and the very opposite of the principles of the parliament. We don't think they have thought through the implications," he said.

SCVO is calling for MSPs to resist calls to broaden the scope of the bill and to insert a clause allowing for the legislation to be reviewed after a year. Meanwhile the register should not be extended to include organisations contact with civil servants, the charity said.

Neil Findlay MSP, who originally proposed a lobbying bill in 2013 said he believed the legislation proposed was too weak and he intended to table amendments to extend it. The Scottish Government’s lobbying bill is a weak and very pale imitation of the Bill I introduced to parliament," he said. "What has been produced is extremely disappointing. I am pleased the majority of the Standards, Procedures and Public Appointments Committee agree that the Bill as proposed needs strengthening."

However he said many charities supported the bill and added: "Charities, some of which operate as big businesses and are in receipt of government contracts, often seek to influence policy and it is right that they are covered by this bill. With the introduction of thresholds the vast majority of charities lobbying would not be included."

He said such thresholds were among the amendments needed to the legislation, but added: "Many charities support the proposals for a register of lobbying. It is just a shame the organisation that leads the voluntary sector in Scotland can't bring itself to support it too."