The riots in England will cost more than £133m in policing and compensation for businesses hit by the violence, the home affairs select committee has been told.

London's mayor Boris Johnson also told the committee that the huge numbers jailed after the unprecedented inner-city violence must not be abandoned in prison, but helped to turn their lives around.

Johnson said he agreed with the justice secretary, Kenneth Clarke, who wrote a Guardian article on Monday revealing that almost 75% of the adults charged across the country with riot offences had previous convictions. He blamed a "feral underclass" and a broken prison system for failing to rehabilitate them.

"One thing I do think the justice secretary is right to highlight is the importance, if you arrest such a huge number of people as we have and you put them into the criminal justice system, then you cannot simply abandon them … you have to make sure they are educated in there," he said.

But while the prime minister, David Cameron, has stressed emphatically that gangs orchestrated the riots, the committee was told by the Metropolitan police that the latest analysis suggested just 19% of those arrested had any gang affiliations. Current analysis also shows that only 21% of those arrested were under 18.

For the first time a clear picture emerged of the costs of the rioting: in London, the bill for policing has reached £74m, and the cost for police forces outside the capital is set to exceed £50m. This includes a bill for Manchester of around £10m – made up of £5m in policing and potentially the same amount in payments under the 1886 Riots Damages Act (RDA) to individuals whose properties are damaged by rioting.

Johnson revealed that in London the Metropolitan Police Authority had already received 100 applications for compensation under the RDA, totalling £9.3m – a figure which is likely to increase – bringing the cost of the riots to around £133m.

Sparked by the fatal shooting by police of 29-year-old Mark Duggan in Tottenham, north London, the riots are now the subject of several inquiries, including the home affairs select committee hearings, an inquiry by the Independent Police Complaints Commission (IPCC) and internal reviews by the Metropolitan police.

The committee was told that the assistant commissioner Lynne Owens is evaluating the police response in Tottenham after Duggan's death, particularly the events outside the police station on the Saturday night when the dead man's family gathered in the hope of speaking to a senior officer. The family heard about Duggan's death from a news report rather than from the police, and had arrived at the station demanding answers.

Godwin said: "There was some confusion in terms of who was going to tell his family. We regret that. A commander has been to see the family to apologise. We need to look at the whole management that took place in Tottenham. Good decisions were taken and there were some misunderstandings and we need to get to the bottom of it."

Godwin said the relationship between the Metropolitan police and the IPCC needed to be examined – referring to an apparent blurring of roles as to who has the duty to inform the family in a police fatal shooting. However, in written evidence the IPCC said it was never its duty to inform a family of a death.

Godwin – who is one of four candidates for the post of Metropolitan police commissioner – told the committee he wished he had had more officers on duty when the riots erupted in Tottenham, on the Saturday evening, before spreading across the capital: "Sometimes you realise how thin the blue line is," Godwin said.

"The debate … would have been different if people had been left in hospital after being seriously injured by baton rounds. I take pride in the fact that we filled up prison places instead of hospital beds, and I think that's the British way," he said.