One of Boris Johnson’s leading supporters has been challenged over whether he has the right character to be prime minister given his past lies and gaffes, as he prepared to appear before the public for the first time in the Tory leadership campaign.

Liz Truss, the chief secretary to the Treasury, said Johnson was being attacked because he was so popular, as she was confronted on the radio with accusations about his misdemeanours.

She also insisted he had nothing to hide despite accusations that he was staying away from public scrutiny and hiding in a bunker until MPs have chosen a shortlist of two candidates to put to the media.

Truss told the BBC Radio 4 Today programme: “Boris is a fantastic person who will be a great leader for our country … he is a positive optimistic person who can help raise our country’s sights.”

The interviewer read an extract from an article by the former Tory MP Matthew Parris, describing Johnson as someone who is an “habitual liar, cheat, conspired to have an offending journalist’s ribs broken, a cruel betrayer of the women he seduces, a politician who connived in court to suppress mention of a daughter he fathered, a do-nothing London mayor and the worst foreign secretary in living memory”.

Truss insisted Johnson was coming under attack because of “the huge public appeal he has and the power he has to communicate”.

She was then challenged over him being sacked from the Conservative frontbench in 2004 by the party’s former leader Michael Howard for lies about an extramarital affair.

She said: “I do not think the British public are interested in Boris’s personal life … there were issues in the past with Michael Howard over his personal life, to me that is not what the public’s priority is here … part of the reason he is getting so much flak is because there are people out there who don’t want us to change. They don’t want the Conservative party to change. They are wrong. If we don’t change, we don’t survive.”

Other candidates have criticised Johnson for avoiding scrutiny, including Rory Stewart, who on Tuesday night cast doubt on whether his rival should be trusted with Britain’s nuclear codes.

At his launch in London on Wednesday morning, Johnson is likely to be pressed on his avoidance of the public as well as his chequered record in office. He is also expected to be asked about a previous admission that he has tried cocaine, after Michael Gove’s campaign became mired in controversy over his use of the class A drug.

Gove’s wife, Sarah Vine, defended her husband in her Daily Mail column on Wednesday, saying he was right to have confessed.

She said it was one lapse of judgment and people should look to his other virtues.

“Most people can find it in their hearts to forgive human error; what they cannot stomach is a barefaced liar. And they are right,” she said.