The BBC said that it was suspending all Newsnight investigations after the programme's accusations that a "leading Conservative" had been involved in child abuse unravelled, with the programme's star witness admitting hours earlier that he had mistaken the peer's identity.

The broadcaster, which is still coping with the fallout from the shelving of a Newsnight investigation into Jimmy Savile, also apologised unreservedly as a senior executive was parachuted in to supervise Friday's edition of the programme, on which the question of its own continuing survival was raised by presenter Eddie Mair.

Steve Hewlett, a Guardian columnist and BBC Radio 4 journalist, also claimed the BBC had investigated Steve Messham, who made the claims about the Tory peer, Lord McAlpine, on at least two separate occasions "and found them wanting".

It also emerged earlier on Friday that the BBC decided it was not appropriate to contact McAlpine, a former treasurer of the Tory party, for a right of reply on Friday of last week because it had no intention of naming him in the Newsnight film. It opted instead to accuse a "leading politician of the Thatcher years" of being involved in child sexual abuse linked to care homes in North Wales.

However, the accuracy of Newsnight's claims collapsed after the Guardian had suggested that McAlpine was a victim of "mistaken identity".

The director general of the BBC, George Entwistle, appointed a senior news executive Fran Unsworth, head of BBC Newsgathering, to supervise Friday night's programme, which carried a full apology.

Referring to its Newsnight programme on 2 November, in which Messham, a former resident of North Wales care homes, appeared and said he had been abused by a political figure, the statement said: "We broadcast Mr Messham's claim but did not identify the individual concerned. Mr Messham has tonight made a statement that makes clear he wrongly identified his abuser and has apologised. We also apologise unreservedly for having broadcast this report."

Entwistle also ordered an immediate suspension of all Newsnight investigations to assess editorial robustness and supervision, a suspension of all co-productions with the Bureau of Investigative Journalism across the BBC, and that Ken MacQuarrie, director of BBC Scotland, will write an urgent report, covering what happened on the investigation into the North Wales children's home scandal.

Friday night's Newsnight was presented by Mair, who normally presents BBC Radio 4's PM, but who has been standing in for more regular anchors such as Jeremy Paxman and Kirsty Wark in recent times. Looking uncomfortable throughout, Mair told viewers: "Obviously we wanted to ask questions of the BBC but no one was available for interview."

However, the most poignant moment came when he was interviewing Rob Wilson, a Tory MP on the Commons culture, media and sport select committee, who momentarily said that he could not hear what he was being asked.

"Oh great. Now not even the sound is working. The journalism is not working," replied Mair, who also later asked Wilson "Is Newsnight toast?"

As well as carrying the recorded comments from Messham and McAlpine's lawyer, the programme included a panel discussion which the BBC press office said had been due to be part of the programme.

On Friday, McAlpine issued a strongly-worded statement saying reports linking him to allegations of abuse at a North Wales children's home were "wholly false and seriously defamatory".

As questions mounted about the veracity of the allegations, Messham admitted he was wrong. Making a public statement of apology, he said he had accused the wrong person: "I want to offer my sincere and humble apologies to him [McAlpine] and his family. After seeing a picture in the past hour of the individual concerned, this is not the person I identified by a photograph presented to me by the police in the early 1990s, who told me the man in the photograph was Lord McAlpine."

Earlier, McAlpine's solicitor said he had "no choice" but to take legal action. Andrew Reid told BBC Radio 4's PM programme: "What I think is so wrong is that Newsnight trailed this and encouraged people that some major revelation would come about and that they were going to name someone.

"Then they took the coward's way out, they ran the programme, then told everyone where to go to find [McAlpine's name in connection with the allegations]. That's creating the defamation."

Newsnight is already under fire for failing to broadcast a previous investigation into child sexual abuse perpetrated by Jimmy Savile – with its editor, Peter Rippon, obliged to step aside after making erroneous claims as to why he chose to abort the story.

Against that backdrop, Newsnight's most recent investigation into child sexual abuse was keenly anticipated as providing an opportunity to make amends.

Organised in conjunction with the Bureau of Investigative Journalism, information about Newsnight's film of last week leaked out the day of transmission when Iain Overton, the editor of the bureau tweeted: "If all goes well we've got a Newsnight out tonight about a very senior political figure who is a paedophile."

The bureau's Angus Stickler, a former BBC journalist, had persuaded Newsnight to put out the film he was preparing. Had Newsnight successfully contacted the peer ahead of transmitting the film, it might have been given McAlpine's denials.

Journalist Michael Crick, of Channel 4 News, who became aware of Newsnight's investigation, spoke to the peer twice on 2 November – the day of transmission – and was told that McAlpine was prepared to sue the BBC, had he been named.

It is understood that the production team at Newsnight had been told in confidence by Messham that McAlpine was the man he was referring to.

The production team, was headed by acting editor Liz Gibbons, and overseen by Adrian van Klaveren, the controller of Radio 5 Live, on secondment to oversee any coverage relating to Savile and child sexual abuse more generally.

Despite the secrecy at the BBC, the name of McAlpine swiftly began circulating on the internet. The peer had previously been linked to allegations of abuse at care homes in North Wales, but several reporters who covered the Waterhouse public inquiry that examined the claims were sceptical of the link.

The Guardian reported that McAlpine was a victim of "mistaken identity" – creating the first doubts about the accuracy of the Newsnight investigation. On Friday McAlpine repeated details that he had shared with Crick a week ago, stating that he had only ever visited Wrexham once in his life.

There was also criticism of people who had named McAlpine on Twitter, including the Guardian columnist George Monbiot, who has since apologised. The journalist admitted it was "stupid" of him to have named McAlpine during a week of fevered internet speculation.