Thousands of prison officers at jails in London and south-east England are to get pay rises of up to £5,000 to boost staffing levels.
The increase means new starters could receive up to £29,500 a year.
Ministers said they wanted to attract the "best talent", after concerns the prison service was understaffed.
Jails have been hit by staff strikes and rising violence in recent months. A union welcomed the rise, but said ministers were "papering over cracks".
The Prison Officers Association (POA) also said the increase was "divisive" as it did not affect all staff.
The pay increases - worth between £3,000 and £5,000 - will apply to "Band 3" staff who joined after 2012. The Ministry of Justice says this is the majority of front-line officers.
A total of £12m will be targeted at prisons in south-east England , such as Wormwood Scrubs, Pentonville, and Belmarsh, which have problems recruiting and retaining their staff.
On Thursday, it emerged that in 2016 the number of front-line prison staff in England and Wales had fallen to 17,888, compared with nearly 25,000 in 2010.
Justice Secretary Liz Truss told the BBC that such numbers were no longer needed.
"The world has changed, we're digitising our prisons, we're running them in different ways," she told Andrew Marr.
A prison service spokeswoman said digital technology allowed officers to focus on "the important task of reforming offenders" - rather than paperwork.
In November, a government White Paper said an extra 2,500 prison officers would be in place by the end of 2018.
The rise includes an extra 400 officers recruited by March this year. The Ministry of Justice said it was "on track" to meet that target, with 389 job offers already made to new recruits.
Shadow Justice secretary Richard Burgon said the Conservative government, and the previous coalition, had made a "political decision" to "decrease frontline prison staff by 6,000".
"That has been the real game changer," he said. "Speaking to prison officers, I know they believe that as well."
The general secretary of the POA, Steve Gillan, said the union had been told about the pay rise on Tuesday, and that "not a lot of thought" had gone into it.
"We welcome any new money," he said, "but we're a national service."
The union says the pay rise will only apply to 31 prisons, out of more than 100 in England and Wales.
Mr Gillan said the increases did not apply to officers on lower grades.
He said: "The lowest-paid people in the service are getting nothing. We pointed that out and there was a deathly silence."
Mr Gillan also said that pay was not the only concern of his members.
"The violence in prisons is out of control," he said. "The prisoners are in control, not the staff."
Ms Truss said: "Prison officers do a challenging and demanding job day in and day out.
"I want front-line staff to know that their work, experience and loyal service is valued.
"We also want to attract the best new talent into the service, ensuring we recruit and retain the leaders of the future."
The 31 prisons involved are: Aylesbury, Bedford, Bullingdon, Coldingley, Cookham Wood, Downview, Elmley, Feltham, Grendon, High Down, Highpoint, Huntercombe, Medway, Send, Stanford Hill, Swaleside, The Mount, Woodhill, Brixton, Belmarsh, Isis, Pentonville, Rochester, Wandsworth, Wormwood Scrubs, Erlestoke, Lewes, Whitemoor, Chelmsford, Guys Marsh and Littlehey.
Ministers are due to publish a bill to reform prison and courts this week.
The government has said the changes will give prison governors more control over jail management, introduce new performance measures and improve prison education.