Many prisoners are “unwell not evil” and should be in secure hospitals rather than behind bars, the chief inspector of constabulary has said as Boris Johnson announces a raft of measures to imprison more people and for longer periods of time.

Sir Tom Winsor, who has been head of the police watchdog since 2012, said “sick and dangerous” people who commit crimes should be in secure hospitals or other care facilities, not prisons, describing the criminal justice system as “dysfunctional and defective, broken and on-the-floor”.

Writing in The Times, Sir Winsor said: “People who are bad should be punished and, if possible, reformed. People who are unwell should be treated and, if possible, healed.

“Of course the public need to be kept safe from those who are dangerous, whether they are bad or unwell. But sick and dangerous people should be in secure hospitals or other care facilities, not prisons.”

It comes after Mr Johnson vowed to create thousands more prison places and “properly punish” offenders – an announcement that prompted criticism from campaigners accusing him of ignoring evidence on causes of crime in order to play to conservative voters.

The prime minister told The Telegraph over the weekend that he was planning to bring in life orders for murderers of pre-school children.

But Sir Tom suggested longer sentences would have little effect, and may possibly have the reverse effect of “making bad people worse”.

“If you brutalise people, they will become brutal. If you care and show in actions that you care, these conditions will degrade and be replaced by hope restored and met, and potential realised,” he said.

“For those in prison – very many of whom are victims as well as offenders – the conditions must be humane, their mental ill health must be understood and properly treated, and their rehabilitation must be properly resourced.

The chief inspector added: “Many people in prison are unwell, not evil.”

The government came under fire last week after announcing that a “mega-jail” is to be built in Yorkshire, paving the way for the first of the 10,000 additional prison places promised by Mr Johnson last month.

Campaigners condemned the decision, saying the new facility would add to the strain on local services, and that expanding a prison system stricken with record levels of violence and despair was not the way to solve the prison crisis.

A Ministry of Justice spokeswoman said: “This prime minister is cracking down on crime and his priority is to keep the public safe. We cannot allow our prisons to become factories for crime. That’s why we’re spending up to £2.5bn to create modern, efficient prisons which reform and rehabilitate offenders, and have invested an extra £100m in airport-style scanners to make our prisons safer.