Surveyors working for the HS2 rail firm have lost documents relating to homes on the Shimmer estate in Mexborough, South Yorkshire.
It has emerged that staff working for Carter Jonas, who are acting on behalf of the high speed rail company, left them on a train.
The company say the information 'is not sensitive' as the documents contain publicly available data on the houses. This includes details of what the residents' surveyor thinks the houses are worth.
Cater Jonas has written a letter to residents apologising for the incident.
The company have reported the incident to the Office of the Information Commissioner - the regulatory body for all matters involving personal information.
British Transport Police and HS2‘s own security advisors have been informed of the incident.
A HS2 spokesperson said they take their responsibility to protect personal data "very seriously."
The incident comes just days after the Government announced that 16 homes would need to be knocked down on the newly built Shimmer estate to make way for the high speed train.
Councillors claim the entire site could be demolished to make way for construction of a 20 metre high viaduct.
They say those unaffected would be in touching distance of the railway.
The Transport Secretary says people in South Yorkshire whose homes will be bulldozed to make way for the HS2 rail line will be guaranteed a 'comparable home.'
Our reporter, Martin Fisher, went to speak to residents on the Shimmer Estate yesterday afternoon.