Call it a lockdown within the lockdown -- U.S. prisoners will be kept inside their assigned cells for the next 14 days in an effort to slow the spread of the coronavirus.

The Federal Bureau of Prisons announced Tuesday night its new guideline will go into effect beginning Wednesday ... calling for all inmates in every prison across the nation to be secured in their assigned cells/quarters to decrease the spread of COVID-19.

This is BOP's Phase 5 plan as it desperately tries to combat the outbreak in prisons across America. BOP also says -- to the extent practicable -- inmates will have access to programs and services offered under normal operating procedures, such as mental health treatment and education.

After 14 days, if BOP deems it necessary ... it can choose to extend the lockdown. BOP will also work with U.S. Marshals to "significantly decrease incoming movement" to its prisons.