Parliamentary authorities are investigating claims that e-mails sent by senior Liberal Democrats, including party leader Charles Kennedy, have been hacked into at Westminster.
One of the two incidents is alleged to have involved a Labour Party worker in the House of Commons.
The hacking was discovered when an internal LibDem inquiry into a policy leak to the Guardian traced the source of the information to a Labour staffer.
The other incident involved the Parliamentary Communications Directorate (PCD) - responsible for the electronic linkage of hundreds of MPs and Commons staff - inadvertently breaching LibDem e-mail security.
The police have been notified and the PCD has launched a full investigation into the allegations.
LibDem Treasury spokes-man Matthew Taylor said he became concerned at the Guardian leak as "only a tiny number of people'' had access to the draft of the document containing part of the party's draft pre-election manifesto.
He added, "We are fairly confident that it was Labour that accessed our e-mail. It can't have been paper copies because they were shredded.''