On Air Now
European Union Election Results 12am - 4am
12 November 2017, 17:56 | Updated: 12 November 2017, 18:05
This caller said Boris Johnson used his mayorship of London to advance a eurosceptic agenda, and couldn't reconcile it.
Last week Boris Johnson was forced to clarify his comments after suggesting Nazanin Zaghari-Ratcliffe was "training journalists" - which could keep her in prison for longer.
He faces an increasing clamour, demanding he resign for the error.
Jonathan, a former colleague of Boris Johnson, said his jaw dropped when he heard Mr Johnson had been appointed Foreign Secretary.
The caller explained how the then Mayor's behaviour alienated him, previously a staunch supporter: "I know Boris better than most, I've worked with him in the past.
"At one point I thought he was the best thing since sliced bread, but I'm afraid I got very quickly disillusioned with him.
"You could see that when he was Mayor of London he was using his position to advance his eurosceptic agenda, which was entirely wrong, particularly in London.
"You have to look at his wife, a barrister. She is a very powerful woman, she was always of the opinion that our laws were being dictated by Brussels.
"I know that some accounts have said Boris wavered between pro and anti. I think he's always been a Brexiteer.
"To me it was quite clear that when he was Mayor of London he was preparing that stance virtually as soon as the referendum was announced.
"And I'm sorry, London is an international city. How can you possibly have a Mayor of London who is advocating Brexit, it's lunatic."
Watch the conversation above.