The justice secretary has said even under the government’s plans to open 14,000 more prison places, they could still run out of space over the next few years.

Shabana Mahmood told the BBC that “building alone is not enough” to deal with the critical lack of space in UK jails.

The government is publishing its 10-year strategy to deal with overcrowding in prisons – including more details of how it will create 14,000 more places in England and Wales by 2031.

It comes after prisons were just 100 spaces away from reaching full capacity over the summer.

The government released thousands of inmates early in order to free up space – with more than 1,700 let out in England and Wales in September, and then another 1,200 in October.

Under its new prisons strategy – which it is calling a “Plan for Change” – the government will build four new prisons over the next seven years, creating 6,400 new places.

The same number of places will also be created by building new blocks on current prison sites.

As well as this, 1,000 makeshift cells with a lifespan of 15 years will also be built – and another 1,000 cells will be refurbished.

“The current state is appalling,” said Mahmood, in an interview with BBC Radio 4’s Today programme being broadcast on Wednesday.

She said the prisons system was “under huge stress” and was seeing prisoners “coming out better criminals than they were before they went in”.

“We’ve got to turn that around,” she said. “I think that the first step in turning things around is dealing with our capacity problems.”

But Mahmood admitted there could still be another overcrowding crisis despite the extra places, and that building more jails was not the only solution.

“We will run out,” she said. “Demand is still rising faster than any supply could possibly catch up with.”

“We’re very honest and transparent in the strategy itself that building alone is not enough because the demand is rising more quickly,” she added.

The prison population has ballooned in recent decades as a result of tougher sentences and court backlogs.

And the number of prisoners has been growing by around 4,500 a year – faster than previous governments had built new cells.

But Carl Davies, from the Prison Governors’ Association which represents managers in the prison service in England and Wales, was cynical about the plans.

“We’ve heard this time and time again: ‘We’re going to build more prisons’,” he said. “Announcements and strategies are great but until we have some certainty, until we see this happen, it’s hard to believe the words.”

As part of the government’s strategy, it is also changing planning rules relating to building prisons.

It said prisons would be deemed sites of “national importance”, placing the power to green-light planning applications solely in ministers’ hands, and with decisions to take no longer than 16 weeks.

The government said £2.3bn had already been secured through the Budget to pay to build the four new prisons, until 2026.

Another £500m will be spent on prison maintenance and the probation service across the next two years.

But His Majesty’s Prison and Probation Service estimates it would cost £2.8bn over the next five years to bring the whole estate into a fair condition, more than double its current maintenance expenditure.

And the National Audit Office warned last week that prison expansion costs were expected to be at least £4.2bn above original estimates in 2021.

As well as this prisons strategy, the government hopes a separate review into sentencing could be a longer-term solution to reduce demand on prisons.

The review of prison sentences is being led by David Gauke, a former Conservative justice secretary.

The review is expected to consider scrapping short sentences and toughening up community orders as an alternative to jail. It will report back next spring.

Asked by the Today programme whether some people currently in prison might not be in the future, Mahmood said it was a possibility.

“Yes… We will have to expand the range of punishment available outside of prison, that does mean that we will have more offenders monitored outside,” she said.

“But I want to make sure that when we are not putting someone in prison but they’re being punished outside, that that is still a punishment they can have confidence in.”

The Conservative government had pledged to build 20,000 new prison places by 2026.

HMP Prison and Probation Service had created more than 6,518 as of September.

The Labour government has pledged to create the remainder – about 14,000 – by 2031, a promise already made during the election campaign

Last week, the National Audit Office released a critical report saying that Boris Johnson’s government pledge to create an extra 20,000 cell spaces was now not expected to be met until 2031 – about five years later than promised. It also said the scheme was running £4.2bn over budget.