Parliament is facing a new expenses scandal after the amount that every MP pays their staff, including their wives and family members, was accidentally leaked.
The Independent Parliamentary Standards Authority is writing to all 650 MPs and their staff to apologise for a "serious data breach" on Thursday night.
Confidential data including the name of every member of MPs member of staff, their exact salary and the amount that they received in bonuses was inadvertently published online.
MPs who saw the data on Thursday night told The Telegraph that the data had the potential to be "hugely embarassing" for MPs who employ their wives, children and other "connected parties".
It comes after the wife of the French presidential candidate Francois Fillon was placed under formal investigation over allegations he paid his family thousands of euros for work they did not do.
Mr Fillon, who was favourite to win the elections, has now slipped behind his rivals. According to official records a total of 150 MPs - equivalent to nearly one in four - employ a "connected party", at a total cost to the taxpayer of £3.6million. Ipsa only publishes anonymised information about salaries and bonuses paid to staff.
The data, in three files, was published on Ipsa's old website for a four hours until the authority was alerted to the breach by Karl McCartney, a Conservative MP.
The independent standards watchdog also published detailed information about each individuals working patterns, holiday entitlements and information about support they receive for any disabilities. There are concerns that the data could have easily been downloaded and could eventually be made public.
Mr McCartney said on Twitter that whoever posted the files online "really needs some IT training before their next job". One MP said the data included information about people's disabilities, with one staff member referred to as "100 per cent disabled".
Ipsa insisted that the information on disabilities related solely to support that staff received. Ipsa said it has referred itself to the Information Commissioner over the incident. Marcial Boo, its chief executive, said: "“We take information security very seriously and the safety and security of MPs and their staff is a priority.
“An investigation is currently under way and we have notified the information commissioner. We will be writing directly to all of those affected. I sincerely apologise to you for the distress this has caused.”
He said no information relating to the security of the individuals affected had been made public.
He said: “No addresses, no bank account details, no phone numbers, and no National Insurance numbers were disclosed. However, we recognise that this was still extremely sensitive personal information."
Ipsa, the independent expenses watchdog, was set up in 2009 after The Telegraph exposed the expenses scandal. It has been unpopular with MPs since its inception.
Chris Bryant, Labour MP a member of the Commons committee which approves its Ipsa's annual Budget, raised concerns that the information is being held by Ipsa in an unencrypted form. He said "I am deeply concerned and I have started writing a letter to the chair of IPSA about it.
"We raised a lot of security issues with them earlier this week but this is obviously worrying. Staff will be more worried than anyone else because they have done nothing to bring themselves into the public domain in this way."
MPs will be banned from hiring their children and spouses on the public purse after the next election after it was criticised as "unjust" and concerns that it encouraged "nepotism".
None of those currently in roles will be forced to resign, with the rule change only applying to new hires after the May 2020 election. An IPSA spokesman confirmed three un-encrypted documents had been published by accident when a staff member was updating documents for MPs to use in the new tax year.