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Key Brexit documents including e-mails from Theresa May were left on a Eurostar in an embarrassing security bungle, it emerged yesterday.

A file belonging to the Prime Minister’s top EU adviser Olly Robbins containing “Cabinet-level briefings” and correspondence with the Prime Minister was left on a Brussels to London train in a major blunder.

“The folder had the summaries on the positions being taken by the Prime Minister and her Cabinet relating to the latest on immigration, customs union and other top-level Brexit-related issues,” an intelligence official said.

(Image: REX/Shutterstock)

“There was also an analysis of what the EU was thinking, as we were trying to second-guess what the EU’s own position would be on Brexit .

“And on top of all that, the paperwork had printouts of email correspondence between Olly and the Prime Minister.

“So it was very sensitive material and could have been very damaging to Britain if it had ended up in the wrong hands.”

The paperwork, which featured Mr Robbins’s handwriting, was found by a retired teacher who alerted officials at the Brexit Department.

Panicked staff rushed to the teacher’s home in Doncaster, South Yorkshire, to collect the documents.

“The retired teacher found the folder on a luggage rack on the train,” a source told the Sunday Times.

“There was an internal investigation at Dexeu (the Department for Exiting the EU) because it was deemed to be an internal security breach.”

The gaffe happened last May.