A new code of conduct passed by MPs yesterday would have kept secret the probe that has resulted in Ian Paisley being banned from the House Commons.
MPs agreed new rules aimed at clamping down on bullying and harassment, but which also ends the power to name and shame MPs and peers accused of other offences such as fiddling expenses or conflicts of interest.
Under the new code, the investigation into Mr Paisley's Sri Lankan holidays would not have been made public.
Since 2010, the Parliamentary Commissioner for Standards has listed all MPs under inquiry on its website and rulings are also automatically published, but the new behaviour code would end this transparency.
Yesterday, all information about two current investigations into two MPs - Labour MP Keith Vaz and Conservative Robert Courts - were removed from the Commissioner's website.
The new code covers MPs, peers and Parliamentary staff.
Commons Leader Andrea Leadsom told MPs: "We're proposing that the Commissioners of both Houses will keep their investigations entirely confidential until such time as there is a finding. This is crucial if individuals are to place their trust in the new system.
"There is clearly a balance to be struck between the public interest in transparency and putting the complainant at the heart of the process by protecting their identity. That is absolutely vital."
But MPs on the cross-party Committee on Standards argued MPs should not have anonymity over accusations of fiddling expenses or conflicts of interest. Committee chairman Sir Kevin Barron said the new code was "a step backwards".
Sir Alistair Graham, the former chair of the Committee on Standards in Public Life, told the Daily Telegraph that it would "seriously undermine our democratic system" and that MPs are "using something relating to sexual misconduct to get rid of something MPs haven't liked for a long time".