Confidential Mexborough HS2 rail documents left on train
Surveyors working on the HS2 rail project lost confidential documents containing personal data of people whose homes are to be demolished.
Residents on the Shimmer estate in Mexborough, South Yorkshire, were told property valuation details were left on a train.
The rail firm is buying some of the newly-built houses to clear them to make way for the high-speed line.
The blunder was slammed by a local councillor as "unbelievable".
HS2 Ltd apologised to residents for the "distress this has caused".
The BBC understands the documents drawn up by Carter Jonas include the name of occupiers, address and valuations of two properties on the estate.
In addition, there is a list of the estimated values of 15 other properties. Some of the information is already publically available.
The residents affected were informed of the blunder by letter on Thursday.
Local independent councillor Bev Chapman described the loss of the documents as "unbelievable".
"What beggars belief even more is the residents have actually been banned from talking about their own valuations to other residents, to their neighbours," she said.
"So now it's gone public, we've got 17 valuations out there in the public domain."
HS2 and Carter Jonas say there are no "requirements that prevent residents from discussing valuations with each other."
In a statement, HS2 Ltd said: "As soon as the loss was reported to HS2 Ltd, we worked as quickly as possible to identify the content of the lost documents and to contact those affected.
"We take our responsibility to protect personal data very seriously. We are working with our supplier Carter Jonas to review what happened and explore whether any further protections can be put in place."