MPs’ staff names, salaries and expenses have been leaked online in a “serious data breach”.
A document containing confidential details about MPs’ staff was wrongly posted on the internet on Thursday night.
The watchdog responsible for MPs’ pay and expenses, the Independent Parliamentary Standards Authority, said they are now investigating what has happened.
The document was uploaded by mistake on an old website, Ipsa said.
It included confidential personal information about MPs' staff names, salaries, rewards, working patterns and holiday entitlements.
In a letter to MPs, Ipsa Chief Executive Marcial Boo said the document was taken down within an hour of them becoming aware of the issue.
He said that it was thought to have been accessible online for a period of around four hours.
"We take information security very seriously and the safety and security of MPs and their staff is a priority," he said.
"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."
Additional reporting by Press Association.