One of the Conservative party’s most generous donors has joined a growing chorus of demands for Boris Johnson to explain why police were called to his home after an altercation with his partner.

John Griffin, the taxi tycoon who has given £4m to the Tories over the last six years, has expressed concerns about the morality of the favourite to become prime minister and called on him to explain the circumstances of a furious row with his partner, Carrie Symonds.

His comments follow demands from Johnson’s leadership rival, Jeremy Hunt, fellow cabinet ministers and Tory backbenchers for the former London mayor to answer questions about his past behaviour amid growing concerns about his character.

Asked about Johnson’s responsibilities before the Conservative party chooses a new leader next month, Griffin, the Tories’ second biggest donor, told the Guardian he should explain exactly what had happened.

“We deserve an explanation about that row, and he has to handle it properly. He can’t assume that we are going to support him when he has not explained every detail,” he said.

“It is likely that he is going to become the PM and most members want to support him. But if I did anything wrong, I would need to explain. Because he hasn’t, it is a real worry.”

The story emerged after a neighbour told the Guardian he had been so concerned by a late-night altercation between Johnson and Symonds that he had felt obliged to call the police. Other neighbours confirmed the argument had taken place and said they had been concerned by its intensity.

Griffin, a Brexiter and founder of the cab firm Addison Lee, expressed concerns about Johnson’s morality and said he should also come clean about his previous behaviour, including his responsibilities to his children.

Johnson has four children by his former wife, Marina Wheeler, and a child with a woman with whom he had an affair. However, he has been dogged by unproven claims that he has at least one more child. Johnson has refused to comment.

Profile Boris Johnson's Tory leadership campaign Show Hide Personal style A late-night altercation between Tory leadership favourite, Boris Johnson, and his partner, Carrie Symonds initially changed the dynamics of Johnson's campaign. He had been either invisible or deliberately sober to the point of dullness, when his usual primary draw to Tory members is a self-created sense of optimism and fun. Much is also made of his supposed broad appeal to the electorate, evidenced by two terms as London mayor. His bizarre claim to make model cardboard buses has raised eyebrows. In most political contests, Johnson’s character – he has lost more than one job for lying, and has a complex and opaque personal life – would be a big issue, but among the Tory faithful he seemingly receives a free pass. It remains to be seen what impact that late-night police visit will have on his chances. Brexit He has promised to push for a new deal while insisting the UK will leave the EU come what may on 31 October, even if it involves no deal. While his hard Brexit supporters are adamant this is a cast-iron guarantee of leaving on that date, elsewhere Johnson has been somewhat less definitive. Asked about the date in a BBC TV debate, Johnson said only that it was 'eminently feasible', although he then went on to tell TalkRadio that the 31 October deadline was 'do or die'. Taxation His main pledge has been to raise the threshold for the 40% higher tax rate from £50,000 to £80,000, at a cost of almost £10bn a year, which would help about 3 million higher earners, a demographic with a fairly sizeable crossover into Tory members. Johnson’s camp insist it would be part of a wider – and so far unknown – package of tax changes. Public spending He has said relatively little, beyond promising a fairly small increase in schools funding, as well as talking about the need to roll out fast broadband across the country. Johnson has generally hinted he would loosen the purse strings, but given his prior fondness for big-ticket projects – London’s cancelled garden bridge, the mooted 'Boris island' airport – perhaps expect more of a focus on infrastructure projects than services. Climate and environment This is unlikely to be a big issue for Conservative party members, and Johnson has not said much on this beyond confirming his general support for the new government target of cutting greenhouse gas emissions to a net zero by 2050. Foreign policy Also unlikely to be a big issue among Tory members, beyond vague platitudes on 'global Britain', it could be a weak spot for Johnson given his poor performance as foreign secretary. He was seen as something of a joke by diplomats – both UK and foreign – and is likely to face more questioning over his gaffe about the jailed British-Iranian woman Nazanin Zaghari-Ratcliffe Peter Walker Political correspondent Photograph: Isabel Infantes/AFP

Griffin, who stood down as chairman of Addison Lee in 2014, said: “Each of his children need his love and attention. But he needs to show that he has given it to them. He cannot say that it is irrelevant. It is highly relevant. It is one of the ways you measure a person.

“He may very well be the best father ever, but he needs to tell us about it. It is a fair question,” he said.

Griffin also called for Johnson to address allegations that he had had extramarital affairs and had treated women in those relationships badly.

“We need to know if he can be trusted because he will get even more attention from women if he becomes PM. I would be concerned if he went marauding around, taking advantage of women by using his position. It would not be right at all,” he said.

Griffin is the second major Conservative donor to demand that Johnson explain what happened on Friday in Camberwell, south London. Another, who has given the party more than £500,000, told the Guardian the issue around the row was damaging the party. “We are a laughing stock,” he said.

Griffin’s intervention comes as the party becomes increasingly dependent upon major donors.

The former mayor of London, who is widely expected to win the leadership race, is due to take part in a digital hustings with Tory party members on Monday.

Earlier on Monday, Hunt sidestepped questions about whether Johnson was a fit person to be prime minister, but emphasised that the runaway favourite among the party’s members needed to earn the trust of voters.

“The way to earn that trust with Conservative party members and with the country is to subject yourself to scrutiny to answer questions about what you actually want to do,” he told BBC Radio 4’s Today programme.

He said it was “incredibly disrespectful” of Johnson to refuse to take part in any head-to-head television debates until after ballot papers had gone out to party members, and warned that a Johnson-led government would rapidly collapse.