Letters from former minister Denis MacShane admitting expenses abuses cannot be used to prosecute him because they are protected by Commons rules, it has been revealed.
Officials said parliamentary privilege meant the key correspondence was withheld from police when they launched a probe into the MP two years ago.
The documents are still not legally admissible - even though they were published in a Commons sleaze report on Friday.
The situation emerged after the Standards and Privileges Committee detailed how Mr MacShane knowingly submitted 19 false invoices over a four-year period. The cross-party group said the invoices were "plainly intended to deceive", branding it the "gravest case" they had dealt with.
The former Labour Europe minister pre-empted the recommended punishment of 12 months' suspension from the House by announcing his resignation as an MP on Friday night. He insisted he had not gained personally from the abuses but wanted to take "responsibility for my mistakes".
"I have been overwhelmed by messages of support for my work as an MP on a range of issues but I accept that my parliamentary career is over," he said. "I appreciate the committee's ruling that I made no personal gain and I regret my foolishness in the manner I chose to be reimbursed for work including working as the Prime Minister's personal envoy in Europe."
A senior Labour source said: "Denis has done the right thing."
Parliamentary Standards Commissioner John Lyon found the MP had entered 19 "misleading" expenses claims for research and translation services from a body called the European Policy Institute (EPI), signed by its supposed general manager. However, the institute did not exist "in this form" by the time in question and the general manager's signature was provided by Mr MacShane himself or someone else "under his authority".
It has emerged that three years ago, at the height of the expenses scandal, MPs voted down reforms that could have made evidence such as Mr MacShane's letters admissible in court. Clause 10 of the Parliamentary Standards Bill made clear that parliamentary privilege should not knock out evidence simply because it related to "proceedings in Parliament".
However, the Commons narrowly voted to drop the measures in July 2009 amid concerns of a "chilling" effect on free speech. A Government green paper on parliamentary privilege launched in April has again suggested changing the rules to ensure such evidence is admissible.