It has been revealed that letters from former minister Denis MacShane admitting expenses abuses cannot be used to prosecute him because they are protected by Commons rules.
Officials said parliamentary privilege meant the key correspondence was withheld from police when they launched a probe into the MP two years ago.
And the documents are still not legally admissible - even though they were published in a Commons sleaze report today.
Mr MacShane announced his resignation after the Standards and Privileges Committee recommended his suspension from the House of Commons after a report detailed how Mr MacShane knowingly submitted 19 false invoices over a four-year period.
He was suspended from the Labour party this morning in the wake of the report.