Last year the government announced it had asked to use 400 cells, following a surge in overcrowding in male prisons and youth jails.
The Ministry of Justice (MoJ) has given the National Police Chiefs' Council) 14 days to make cells in the north of England and West Midlands available.
Neither body would confirm how many had been requested.
The Press Association said it understood it may be in the region of about 100.
In November, justice minister Damian Hinds said there had been a "highly unusual" surge of more than 800 prisoners over two months.
The MoJ has described an "unprecedented increase" in the number of offenders coming into prisons in the north of England.
On Monday, it gave the National Police Chiefs' Council (NPCC) two weeks to make cells available to hold prisoners, after there had been another rise in the number of inmates since the start of the year.
An MoJ spokesman said: "We have given notice to the National Police Chiefs' Council to make available cells in police custody suites in the north of England and West Midlands as planned under Operation Safeguard.
"This will help ensure we have enough spaces to manage the short-term pressure on prison places."
An extra 20,000 prison places are being built, with the newest jail set to open in the spring, the spokesman added.
Critics previously warned the problem could have been predicted, while others claimed the plan could put the public and police officers in danger.
An initial £14m has been allocated for the scheme, based on estimations it will be needed for three months, and the funds will be taken from the MoJ's existing budget.
But the total cost to the department will depend on the number of cells occupied and how long for.
When he made the announcement, Mr Hinds described Operation Safeguard is an "established protocol", which has been used before in periods of "high demand" including between 2006 and 2008.
He stressed the country had "not run out of prison places" and the emergency measures - which will provide the "immediate additional capacity" needed - "do not reflect a failure to plan ahead".
As of Friday, the prison population stood at 83,188 with a "useable operational capacity" of 84,607, indicating close to 1,500 spaces are available.
It was understood some spaces were always kept free so prisons had the capacity to operate safely and respond to any unforeseen circumstances, PA said.