A top UN official has made an outspoken attack on the government's controversial lobbying bill, describing it as a "stain" on democracy that will undermine elections in the UK, as leading charities demand fresh concessions on the proposals from coalition ministers.

Before key votes on the bill in the House of Lords this week, Maina Kiai, the UN rapporteur on rights to freedom of peaceful assembly and association, says the legislation, if not amended further, will reduce the ability of people in civil society to express their views before elections, while doing little or nothing to tighten controls on corporate lobbyists.

In an article published on Sundayon this newspaper's website, Kiai, a Kenyan lawyer appointed by the UN's human rights council, said: "Although sold as a way to level the electoral playing field, the bill actually does little more than shrink the space for citizens – particularly those engaged in civil society groups – to express their collective will. In doing so, it threatens to tarnish the United Kingdom's democracy."

He wrote: "Provisions ostensibly designed to target corporate lobbyists have a loophole so big it swallows the rule. In-house lobbyists – which enjoy the most influence in the UK government by far – are exempt. That leaves unions and civil society as taking the brunt of the bill's impact."

His intervention will embarrass the government, which has been criticised for shoddy drafting of the bill and seeking to silence organisations critical of its policies, from charities to trade unions, while leaving the lobbying industry largely unaffected.

Last week ministers, under intense pressure from charities, announced a series of amendments to the bill, which aims to regulate election campaign spending by those not standing for election.

It would also introduce a statutory register of "consultant lobbyists", and place new requirements on trade unions to keep lists of members.

In a series of climbdowns, ministers dropped a plan to cut the amount charities in England could spend on campaigning before they had to register with the Electoral Commission from £10,000 to £5,000. This will now be raised to £20,000. The proposed overall spending limit for charities across the UK during the regulated period is also being raised from £390,000 to £450,000.

But while the concessions were welcomed, more than 75 charities including Oxfam, the Countryside Alliance, Amnesty International, the Salvation Army and the National Federation of Women's Institutes, have since launched a new petition demanding more changes to prevent the bill having a "chilling effect" on their ability to campaign.

The charities remain fiercely opposed to limits the bill would impose on what they could spend in any single parliamentary constituency. They also want staff costs exempted from the limits. Some argue that charities are already covered by rules banning them from political activity and therefore should be excluded from the bill altogether.

In a letter to the Observer, the former bishop of Oxford, Lord Harries of Pentregarth, a crossbench peer who chairs a special commission set up to advise ministers on regulation of civil society in the runup to elections, said last week's changes to the bill were "important" but did not go far enough to address "the strength of concern that remains".

He said the bill would still "limit the right of charities and campaigning organisations to speak out on some of the most important issues facing the country and planet ahead of elections".

Peers are warning of a cross-party revolt unless the government backs down further. The section of the bill covering lobbying has infuriated many peers because the proposed register of lobbyists would only include lobbying companies, while exempting individual lobbyists or those working "in house" for a company. Only meetings with ministers and the very highest civil servants would be published, not those with other mandarins or special advisers.

A Cabinet Office spokesperson said the bill would bring transparency and accountability to the political system and that organisations with no electoral agenda would be unaffected.

"The government has listened to the concerns which campaigners have expressed. We have tabled important amendments to the bill," the spokesperson said. "These amendments directly address the concerns raised while preserving the core purpose of the bill."