Opponents have effectively scuppered a bill which would exempt MPs from Freedom of Information Act inquiries
The bill protects MPs' correspondence from release
Ex-Tory chief whip David Maclean had brought in the private member's bill.
He says he does not want letters on behalf of constituents published - but it would also curb requests about issues such as MPs' expenses claims.
Lib Dem MP Norman Baker and a handful of MPs from all parties managed to talk out the planned bill by making sure the debate continued for five hours.
This means the bill now goes to the bottom of the queue for private member's bills and has virtually no chance of becoming law unless it gets government backing.
The two-clause bill would effectively remove both the Commons and House of Lords from the list of public authorities obliged to release information under the 2000 act, which came into force in 2005.
It also protects all MPs' correspondence from release and stops authorities being able to confirm or deny whether they have received a letter from an MP.
Mr Maclean told MPs at an earlier stage: "When we write on behalf of constituents... we must be able to look them in the eye and say that in all circumstances what they tell us will not get out.
"It is like the relationship with a priest. We will write to an authority with their problem, but we guarantee that that information will not be leaked by us, or get into the public domain."
Data Protection Act
He said Speaker Michael Martin had assured him Parliament would still publish general details of MPs' expenses and allowances, as now, even though it would not be legally obliged to do so.
Mr Baker, who successfully fought a two-year Freedom of Information battle for a detailed breakdown of MPs' travel expenses, said that a constituent's inquiry would not be "leaked" as it was already covered by the Data Protection Act.
"There is no question that this already exists as a proper means of protecting constituents," he told BBC Radio 4's Today programme.
Of the Bill he said: "This is not about constituents' correspondence, this is about exempting MPs from scrutiny in the House of Commons on how, for example, we get our expenses."
He added: "It's about covering up and it shows, I'm afraid, that the Freedom of Information Act culture that we hoped was becoming established in this country, is not actually in the bloodstream yet."
The bill gained its second reading in the Commons in January and also got through its committee stage, meaning it has got further to completing its Commons stages than most private members bills.
However it was "filibustered" by Mr Baker, who personally spoke for about two hours, and colleagues as they managed to keep the debate on the bill - and a large number of amendments - going from its 0930 BST start to the cut-off deadline of 1430 BST.
Ironically, the first delaying tactic was to call for a time-consuming vote - which did not succeed - on whether the whole debate should be conducted in private.
In a separate move, ministers are attempting to limit the amount of resources spent on freedom of information requests to £600 - including officials' time.
The Department for Constitutional Affairs is running a consultation exercise on the proposal until 21 June.