Confidential personal details about MPs' staff and their salaries has been mistakenly published by the expenses watchdog.
Independent Parliamentary Standards Authority (IPSA) said information about staff rewards, working patterns and holiday entitlements was briefly available on its website on 30 March.
It insisted that no addresses, bank account details or phone numbers had been given out.
In his letter to MPs, IPSA Chief Executive Marcial Boo said: "I would like to reassure you that no information relating to the security of the individuals affected was made public - 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."
The letter ends: "I sincerely apologise to you for the distress this has caused."
The watchdog has apologised and launched an inquiry.