Labour is prioritising pay rises for “union paymasters” over keeping the public safe, Robert Jenrick has warned.

The Government wants to build four new prisons in seven years to ease the overcrowding crisis behind bars.

But Mr Jenrick, the Shadow Justice Secretary, claimed the new Prison Capacity Strategy does not include “any money for new prisons”, adding that career criminals are declaring they are “Labour voters for life”.

Justice Secretary Shabana Mahmood said the new jails will create an additional 6,400 places by 2031, costing £2.3billion.

On Wednesday, Ms Mahmood will confirm plans to generate another 6,400 places by expanding cell blocks in some of the existing institutions.

Ministers are expected to force the new prisons on communities across the UK by changing planning laws to declare them as “sites of national importance”.

This will prevent furious locals from blocking the plans, it is understood.

The Ministry of Justice (MoJ) also confirmed it will buy up more land “for potential future prisons” in case demand continues to outstrip supply.

Ministers are relying on a combination of sentencing reforms – which will mean fewer criminals sent to jail – and more prison places to prevent a repeat of the crisis this summer. At one point there were only 100 empty cells left.

But Mr Jenrick said: “Despite all the fanfare, Labour are not announcing any money for new prisons. Reeves could fund inflation-busting pay rises for her trade union paymasters, but not new prisons to keep the public safe.

“Labour could be maximising court capacity and expediting the deportation of foreign criminals, but instead they are rushing to release prisoners. It’s little wonder career criminals pop champagne and declare that they are now Labour voters for life.’’

However, Ms Mahmood said: “The last government pretended they could send people away for longer and longer without building the prisons they promised.

“This strategy reveals that their prison-building plans were years delayed and nearly £5billion over budget. They left our prisons in crisis, on the edge of collapse.

“Part of our plan for change, this capacity strategy, alongside an independent review of sentencing policy, will keep our streets safe and ensure no government runs out of prison places again.”

The announcement comes after government estimates published last week indicated more than 100,000 prisoners could be held in jails in England and Wales by 2029.

Since September, thousands of inmates have been freed early in a bid to cut jail overcrowding by temporarily reducing the proportion of sentences which some prisoners must serve behind bars in England and Wales, from 50% to 40%.

But prisons are still expected to reach critical capacity again by July.

MoJ figures show there were 86,089 adult prisoners behind bars in England and Wales on Monday.

The so-called operational capacity for English and Welsh men’s and women’s prisons is 88,822, indicating there is now cell space for 2,733 criminals.

The National Audit Office (NAO) last week warned prison expansion plans are “insufficient to meet future demand” amid a projected shortage of 12,400 prison places by the end of 2027, with costs expected to be at least £4billion higher than initially estimated.

The watchdog also said the then-government’s 2021 pledge to create an extra 20,000 cells spaces by building more prisons, temporary wings and refurbishing existing cell blocks is now not expected to be met until 2031 – about five years later than promised.

As of September, a third (6,518) of the 20,000 had been made available, according to the NAO.

Reasons behind delays include “unrealistic timelines” and overestimating the ability to get planning permission for three out of the six new prisons due to be built.

The plans are now expected to cost between £9.4billion and £10.1billion – least £4.2billion more than 2021 estimates amid a rise in construction prices, the report said.

The NAO also warned the government department “does not have any contingency plans to increase prison capacity as it views it has limited options left to do this”.

Ministers have admitted that fewer criminals will be sent to prison in future years because of the overcrowding crisis.

Changes to sentencing will end short custodial terms and ensure more criminals are punished in the community through house arrest and greater use of technology such as electronic tagging to restrict their movements.

Ms Mahmood has said she wants to “reshape­ and redesign what punishment outside of a prison looks like”.

A MoJ source said last week: “This damning report sets out the true scale of the inheritance we were left with. We will build the places the last government did not. But, as this report shows, we cannot build our way out of this crisis, and this sets the terms for the sentencing review.”

Ms Mahmood said on Tuesday that the Government’s independent review of sentencing would serve as a separate way of freeing up places.

The shortage of available places for prisoners led to thousands being released early after spending 40% of their jail term behind bars.

Speaking during justice questions in the House of Commons on Tuesday, she said: “Even with the new supply that we are building, we are still going to run out of prison places because the demand into the system is much higher than the building that is being planned.

“We simply cannot build our way out of this problem, so in order to make sure there’s always a prison place for the people that need to be locked up and that we never run out of prison places again, we do have to have an independent review of sentencing.”

Chairman of the Justice Committee, Andy Slaughter, said some of the UK’s existing prisons are “little more than a human warehouse”.

The Labour MP for Hammersmith and Chiswick said: “The condition, especially of our Victorian prisons, is not conducive to rehabilitation or preparation for life on release. The Government is pressing ahead with the construction of 20,000 new prison places, which their predecessor failed to honour.

“What thought in the design and operation of these major new prisons has been given to the training, education, addiction and mental health needs (of) inmates for whom prison is currently little more than a human warehouse?”

Justice minister Sir Nicholas Dakin replied: “Obviously these new prisons will be built with all the things he mentioned taken fully into account, and the Government is determined to put 14,000 places in place.”