Prison officers at 31 jails in London and south-east England are in line for pay rises of up to £5,000, under a £12m package announced by the government.
The increase means new starters could receive up to £29,500 a year.
Ministers said they wanted to attract the “best talent”. Prisons with recruitment issues are being targeted.
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) added that the government was dealing with “crisis management on a daily basis”.
The pay increase applies to “band 3” staff, who make up the majority of front-line officers.
The Ministry of Justice said “thousands” of employees would benefit. The £12m package is an attempt to boost falling prison officer numbers.
On Thursday, it was revealed that, in 2016, the number of front-line staff in England and Wales fell by 347 (1.9%) to 17,888.
The leaving rate was almost 9% – almost double the level of four years earlier.
Steve Gillan, the general secretary of the POA, said it had been told about the increase on Tuesday, and that “not a lot of thought” had gone into the rise.
“We welcome any new money,” he said, “but we’re a national service and this only applies to 31 prisons [out of more than 100 in England and Wales].
“It doesn’t apply to the operational support grades (OSG’s), so 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.
In November a government White Paper announced an extra 2,500 prison officers would be in place by the end of 2018.
That was on top of an extra 400 officers, to be in place by March this year.
The Ministry of Justice said it was “on track” to meet that target, with 389 job offers made to new recruits.
Justice Secretary Liz 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.”
Mr Leech said: “An extra £12 million is a significant amount to have prised out of the Treasury – but it equates after tax to less than an extra £80 a week for those officers in the most difficult jails.
“The fact that it doesn’t apply to OSG’s is regrettable, they do not have main prisoner facing roles that is true, but it hardly makes for a cohesive workforce.
“It is unclear whether the extra money will be enough to make the most hard-pressed prison officers remain in the job, but it is a welcome step in the right direction.”
The 31 prisons affected 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.