A builder reportedly sparked a security alert this week when he left important plans in a Greggs bakery.
Bakers contacted police on Wednesday to hand over the Parliament papers after discovering them inside the Westminster branch.
The paperwork included detailed maps of the Palace of Westminster, which are restricted for security reasons, according to The Sun.
A source said: "As soon as they realised what it was they handed it in – luckily no member of the public saw what was inside."
Contractors realised the mistake the following morning, only to be told by staff at the branch that the plans were now with police.
A House of Commons spokesperson said: "Documents were left by a contractor outside of the parliamentary estate but have now been returned.
“The House of Commons takes the matter seriously and is currently investigating this incident.”
The Houses of Parliament are currently undergoing building work and upgrades.
Greggs reportedly declined to comment when approached by The Sun.
The Met Police said: "On 12 August, a police officer at the Palace of Westminster took receipt of documents found by a member of public.
"These were passed to staff at the Palace of Westminster who are investigating the incident."
