The BBC Trust found that, although Lord McAlpine denied the allegations when he was contacted by a Channel 4 News journalist before Newsnight was broadcast, the BBC still made no attempt to contact the peer, which it should have done.
Trustees also said that concerns had been raised previously about the credibility of some of Mr Messham's evidence - which was known to members of BBC Wales, but no effort was made to contact them.
The Trustees considered that parts of BBC News not contacting colleagues in the nations and regions when investigating stories on their patch was very poor practice.
More top news
The traffic on British roads hit its highest ever level last year, with congestion continuing to rise, official figures show.
A number of flood alerts are in place for the tidal stretch of the River Thames after it burst its banks yesterday.
The NHS could save over £8 million a year on dental treatment if all 12-year-olds chewed sugar free gum after meals, a study claims.