Hundreds of prison workers are deciding whether to walk out in a pay row after receiving strike ballot papers for the first time in 25 years.
Officers get better pay than other prison workers, Amicus claims
The union Amicus said the staff were furious over the terms of a pay deal which amounts to 1% for most of them.
It claims prison officers have received a better deal than the support staff it represents, who felt "undervalued".
Amicus boss Paul Reuter said: "This is about equal treatment. Our members feel they have been treated inequitably."
He added: "We want the prison service to return to the negotiating table and reach a settlement that recognises the crucial contribution our members make to the prison service."
Around 1,500 industrial staff working in public sector prisons across England and Wales are being asked to vote on whether to take strike action, Amicus said in a statement.
Amicus, which says it is the largest union representing industrial staff in the prison service, called the 1% pay offer "derisory".
The Prison Officers' Association was unavailable for comment.
The move comes as nearly 100,000 civil servants in three government departments are striking in a row over low pay.
The two-day walkout, which began on Wednesday, is expected to affect Jobcentres, benefit offices and the Child Support Agency.
Other workers from the Prison Service are also observing a 48-hour stoppage in a separate wage dispute which could disrupt the transfer of prisoners.
Employees at the Office for National Statistics (ONS) are also staging their first strike in history, as part of a pay dispute.