So, who to believe? In what was ultimately an interview about trust and integrity, the aftermath of Boris Johnson's embarrassing televised grilling at the weekend only led to more questions about the sincerity of the London Mayor's words, as his own father signalled that his attempts to appear unflustered and amused by his experience may not be entirely genuine.

After being forced to deny he was a "nasty piece of work" during surely the most humiliating interview of his life on BBC's The Andy Marr Show, Mr Johnson congratulated stand-in presenter Eddie Mair for his "splendid job" that should win a Pulitzer Prize.

Whether Mr Johnson would say the same thing privately came into question, however, when the 72-year-old Stanley Johnson launched a rhetorical broadside on the broadcaster for what he called a "disgusting piece of journalism" on his son. Their reactions could not have been more contrasting after the Mayor came up against long-standing allegations, raised by Mair on Sunday, that he had been sacked from The Times for making up quotes, lied about an affair and once promised to get the address of a journalist so an old school friend could get them beaten up.

The man himself declared: "Eddie Mair did a splendid job. There is no doubt that is what the BBC is for – holding us to account.

"He was perfectly within his rights to have a bash at me – in fact, it would have been shocking if he hadn't. If a BBC presenter can't attack a nasty Tory politician what's the world coming to?"

His father struck a different tone. Appearing on the London talk-radio station LBC, Mr Johnson Snr – himself a seasoned Tory stalwart – said of Mair: "His grilling people about their personal lives, accusing them of guilt by association, openly abusing them in a legitimate interview. Frankly, I don't know where we are coming to." Speaking to presenter Nick Ferrari, he continued: "I have no idea who Eddie Mair is or what he does. But frankly, there is such a thing as respecting the office, even if you don't respect the man and that did not come through.

"As for saying he thought Boris was a nasty piece of work, well, honestly. I don't know where Eddie Mair's politics come from but I suspect he would not have treated the leader of the Labour Party in that way."

It is suggested that Mr Johnson's father had either not apprised him of what he was intending to say – or it was part of a dual-pronged strategy to laugh off and attack an interviewer who managed to make the Mayor appear uncharacteristically flummoxed.

Asked whether Mair should get Jeremy Paxman's lead anchor role on Newsnight, the Mayor worked hard to maintain his diplomatic and good-humoured stance: "I should think he'll get an Oscar, it was an Oscar-winning performance. I think he'll get a Pulitzer." He added: "I fully concede it wasn't my most blistering performance, but that was basically because I was set to talk about the Olympics and housing in London and he wanted to talk about other things, some of them – my private life and so on – of quite some antiquity, the details of which I wasn't brilliant on."

The BBC defended Mair and its journalism from Mr Johnson's praise and his father's condemnation. A BBC spokeswoman said: "We believe this was a fair interview which took in issues facing London and the wider political landscape. Eddie's line of questioning attempted to elicit responses to direct questions that were not being answered."

Boris laid bare truth or lie

Claim 1: That he lied to Tory leader Michael Howard about allegations of an affair in 2004.

What he said: "I never had any conversation with Michael Howard about that matter."

The reality: Boris may not have lied to Mr Howard, but he publicly claimed the allegations were an "inverted pyramid of piffle". This was not the case – forcing his departure.

Claim 2: That he promised to get the address of a journalist whom an old friend wanted to have beaten up.

What he said: He stressed "nothing eventuated" from the conversation, adding: "I think if any of us had our phone conversations bugged people say all sorts of fantastical things whilst talking to their friends."

What happened: Mr Johnson certainly made the promise – but he was also right to point out that in the event the reporter was never hurt.

Claim 3: That he was sacked as a reporter on The Times for making up a quote.

What he said: "I mildly sandpapered something someone had said and it was very embarrassing and I'm very sorry about that."

The reality: Mr Johnson has previously admitted that he phoned up his godfather, the historian Sir Colin Lucas, for a story he was working on. He also admits misquoting him in a way that jeopardised his academic reputation.