More than 40 police cells in the West Midlands have been made available in case, as predicted, prisons in England and Wales reach capacity. Featherstone Prison near Wolverhampton has just four places left, while HMP Birmingham, in Winson Green, is already completely full.
West Midlands Police is preparing for any overspill by providing 44 cells to hold inmates temporarily.
The Home Office said capacity is set to rise to 80,400 by the end of the year.
The Prison Officers Association is calling the situation a crisis and is demanding talks with the Home Office.
Last week, prison governors said jails in England and Wales were at "bursting point", as figures showed there were currently only 160 spaces available.
Home Secretary John Reid has been discussing options such as deporting some of the 10,000 foreign prisoners.