The House of Commons will vote later on plans to establish an independent panel of experts to deal with bullying allegations against MPs.
The panel would replace the committee of MPs that currently has the final say on action taken.
But the FDA union says there are concerns over plans to hold debates on the panel’s recommendations.
It says fear of the debates would prevent staff from coming forward and it urged MPs not to back the proposal.
In 2018 an inquiry led by Dame Laura Cox recommended that the process to deal with bullying, harassment and sexual harassment complaints should be independent of MPs.
The expert panel would be made up of eight members who are “independent of Parliament” – so no MPs – and it will be able to impose sanctions and hear appeals.
But if it proposes the suspension or expulsion of an MP, the House will consider a motion to impose the sanction.
Writing in The House magazine, Amy Leversidge, assistant general secretary of the FDA Union said: “Fear of a debate among MPs risks deterring staff from putting in complaints, particularly in cases involving sexual harassment, and staff will clearly be concerned about how the proposed rules can meaningfully be enforced when MPs are protected by parliamentary privilege.
“There will be little faith that these proposed rules will be abided by because if MPs intended to abide by them, they wouldn’t be asking for the right to debate at all.”
More than 107 people have signed a petition urging MPs not to agree to debates on the panel’s decisions.
Enya Evans, who started the petition, wrote: “If this motion passes unamended… many staff – myself included – will be dissuaded from ever filing a complaint about a Parliamentarian.
“No one is ever going to complain about them if they think their experiences are literally going to be debated by the perpetrators.”
Conservative MP Andrea Leadsom and Labour MP Chris Bryant have both put forward suggested changes – amendments – to the motion, which aim to limit the opportunity for MPs to take advantage of debates to attack accusers.
Mrs Leadsom wants to move the opportunity to debate from the floor of the House to within the Standards Committee, while Mr Bryant has proposed ditching the debate and sending it straight to a vote instead.
Dame Laura Cox’s report, published in 2018, detailed alleged sexual harassment by MPs from women who said they were “inappropriately touched” and “repeatedly propositioned”.
“Disturbing” cases were “long been tolerated and concealed”, the report from the former High Court judge said, adding that senior House of Commons staff needed to make radical changes.
It was followed by other reports, including Gemma White QC’s on bullying and harassment of past and present staff of MPs, as well as MPs themselves, and an independent review of Parliament’s Independent Complaints and Grievance Scheme.