Prison reform campaigners have slammed Boris Johnson’s proposal to “come down hard” on violent and sexual offenders as a bid to win votes.
The Howard League for Penal Reform said the Prime Minister’s comments were political “hot air” and would not make a real difference to the justice system.
Chief executive Frances Crook said there was no evidence to suggest longer sentences would reduce crime.
“We have excessively long prison sentences already and it doesn't seem to be keeping us safe,” she said.
“What's coming out of Number 10 is politics but not real life. It's not going to deal with real-life crimes and victims. It's a lot of hot air.”
She added: “I have seen politicians trying to whip up a lynch mob mentality for their own benefit before and that does appear to be what's happening now.”
Mr Johnson said criminals must be taken off the streets and punishments should “truly fit the crime” to encourage confidence in the justice system.
“Dangerous criminals must be kept off our streets, serving the sentences they deserve - victims want to see it, the public want to see it and I want to see it,” he said.
He has promised £2.5 billion to create 10,000 more prison places and £85 million to help the Crown Prosecution Service manage its caseload.
The pledges have fuelled speculation Mr Johnson is preparing for a snap election in the event of a no-deal Brexit.
Prison Reform Trust director Peter Dawson said any prison reform must begin with “facts” and “stoking up public anxiety” is a poor way to start.