LIB-DEM MP for Torbay Adrian Sanders will today have to apologise to Parliament after the leaking of a secret document.

And the MP's researcher is set to be barred from Westminster for 28 days for "serious" contempt in passing on confidential information, and then trying to cover it up.

It follows an investigation by the parliamentary sleaze watchdog, which was called in after extracts from a draft report by the Culture, Media, and Sport Select Committee appeared in an article on the Guardian's website.

The Committee on Standards and Privileges found no suggestion that Mr Sanders, who sits on the select committee, was directly responsible for the leak or involved in the unauthorised release of sensitive papers. It also noted his strong opposition to the practice of leaking, which in evidence to the inquiry Mr Sanders condemned as "reprehensible".

But members of the cross-party committee concluded he was responsible "in a general sense" for the actions of his researcher, Tom Smith, who, unbeknown to Mr Sanders, was regularly passing on privileged documents to another researcher, which led to substantial parts of the draft select committee report into the BBC's commercial operations being published.

Although Mr Smith's contract contained confidentiality clauses, the inquiry found no evidence Mr Sanders had explained the importance of keeping select committee papers confidential, or the consequences of failing to do so.

Members concluded that the MP was "insufficiently attentive" to his duty of care to staff, and had failed in his responsibility to ensure the security of all committee papers. As such, it recommended he apologise for these "oversights" in the Commons.

Mr Sanders is expected to say the inquiry's findings served as a lesson to all MPs, particularly members of select committees. He will tell Parliament: "The Standards and Privileges Committee clearly feels a specific duty of care rests on each of us to explicitly make these matters clear to every member of our staff, and I suspect many Hon Members will not have done so presently. In so far as I failed to make this plain to a member of my staff, I of course accept the conclusions of the committee and apologise to the House."

He will suggest new guidance be circulated among MPs and all staff.

Meanwhile, the Standards and Privileges Committee accused Mr Smith of misleading the committee in attempting to cover up his role in the leaking of the sensitive document.

"If he did not lie, he certainly dissembled," the report concluded. The committee would have proposed a more severe sanction had it not been for mitigating factors, including the fact that the document was not marked to indicate its sensitivity, and that Mr Sanders had not informed him of the need to ensure the security of all papers.