The Government is expecting to recruit up to 100 reserve prison officers to work in jails across England and Wales, sources said.
Justice Secretary Chris Grayling announced plans for a "reserve capability" earlier this month after admitting some prisons were experiencing staff shortages, partly blaming the shortfall on an increase in the number of other jobs available.
A Whitehall source said a pool of up to 100 reservists is now being sought on short fixed term contracts of between three and nine months.
They will be deployed nationally "where an operational need exists", the source said.
Staff who left through an voluntary early departure scheme will be eligible but will be expected to repay any payments received.
They will be paid a pro rata salary based on the number of hours specified in their contract and will be subject to the same terms and conditions that generally apply to other staff occupying these roles, the source said.
The BBC said the Ministry of Justice (MoJ) has sent a letter to about 100 ex-prison officers to gauge interest in the "HM Prison Service Reserve" initiative.
One prison officer who received the letter told the BBC: "The reason for this 'reserve force' being considered is the critical shortage of prison officers in prisons up and down the country."
Earlier this month, Mr Grayling denied there was an "overcrowding crisis" in prisons as he responded to an urgent question from shadow justice secretary Sadiq Khan.
Mr Khan had warned that jails have "unsafe warehouses" where prisoners are not working or going on courses due to staff shortages.
Addressing the Commons earlier this month, Mr Grayling said: "I am taking steps to address what I believe is a weakness in our prison system: the fact that we have no access to the kind of temporary or agency staff routinely found in our health and education systems.
"I am establishing a reserve capability among former staff to give us the flexibility to adapt to short-term changes of population by bringing reserve capacity into operation.
"We currently have some staff shortages in London, particularly because of the rapid improvement in the labour market, and this step will help us to cover any gaps."
Juliet Lyon, director of the Prison Reform Trust, said: "Not content with dismantling a functioning probation service, it seems that the Justice Secretary is prepared to allow our prisons to be staffed by a rag-tag army of short-term reservists. This risks trashing rather than transforming rehabilitation."