The Daily Telegraph has been rebuked by the press watchdog after its reporters posed as constituents and secretly recorded Liberal Democrat ministers criticising their Tory coalition partners.
The Press Complaints Commission (PCC) ruled that it was not justified in using high levels of subterfuge and had mounted a "fishing expedition" in the hope that the MPs would make newsworthy remarks.
The Liberal Democrats faced acute embarrassment after the undercover reporters taped ministers including Vince Cable, Michael Moore, Ed Davey, Steve Webb and Norman Baker making disparaging remarks about the Conservatives.
Tim Farron, the party's president, lodged a complaint in which he accused the paper of setting out to entrap MPs.
The Telegraph argued that it been told, both by MPs and readers, that prominent Liberal Democrats were saying one thing in public and another in private. It maintained it had been justified in bringing the disparity into the open.
In a strongly worded ruling published today, the PCC said that the paper had breached guidelines under the Editors' Code of Practice governing the use of clandestine devices and subterfuge. It acknowledged there was a broad public interest in the issue pursued by The Telegraph, but it concluded it had been wrong to focus on "what amounted to disproportionately intrusive attention on a number of MPs (who had been selected purely on the basis of their ministerial position)".
It said that ministers had been asked by the reporters "to comment on a series of policy issues with the evident intent of establishing on which subject they might say something newsworthy".
The Liberal Democrats welcomed the ruling, saying: "It's an important principle that MPs should be able to discuss things with their constituents without being recorded." Mr Farron said:"Everyone should be able to go an MP's office and expect that the meeting will be in confidence, open and honest."