The US Navy has proposed constructing tent cities on abandoned airfields to house thousands of immigrants detained under President Donald Trump’s hard-line immigration policies, according to a new report.

A Navy draft memo reportedly identified five locations for the “temporary and austere” tent cities, which could house nearly 50,000 people. The Navy reportedly drafted the plan in anticipation of requests from the Departments of Homeland Security and Health and Human Services, after Mr Trump ordered them to assist in housing migrant families detained under his “zero tolerance” immigration policy.

The memo suggests a possible 47,000-person tent city at a former airfield outside San Francisco, according to Time, which first obtained the document. Another possibility is Camp Pendleton, a military training facility on the Southern California coast.

Two more possible locations, housing 25,000 people each, have reportedly been identified at abandoned airfields in Alabama. The memo also suggests further study of housing opportunities at a Marine Corps Air Station near Yuma, Arizona.

The numbers greatly exceed the 20,000 migrants the Defence Department said it was preparing to house on military bases this week. That plan – announced by Pentagon spokesman Lieutenant colonel Michael Andrews on Thursday – was condemned by immigrants' rights advocates, who felt it was a hastily constructed solution to the migrant crisis.

Trump's US immigration policy explained

The memo estimates it would cost about $233m (£176m) to construct and operate a 25,000-person facility for a six-month time period, according to Time. It suggests the tent city should be built to last between six months and a year. It also estimates that construction of a 5,000-person facility would take 60 days, with 10,000 more people being added each month.

The memo was reportedly written by Phyllis L Bayer, the assistant secretary of the Navy for Energy, Installations and Environment. It recommends Navy secretary Richard Spencer sign off on the plan and send it to defence secretary James Mattis for approval.

Captain Gregory Hicks, the chief Navy spokesman, told The Independent it would be “inappropriate to discuss internal deliberative planning documents”.

A spokesperson for Camp Pendleton referred requests for comment to the Department of Defence, which did not respond. The Marine Corps Air Station Yuma also did not respond to requests for comment.

Undocumented immigration across the US-Mexico border Show all 14 1 /14 Undocumented immigration across the US-Mexico border Undocumented immigration across the US-Mexico border Immigrant children, many of whom are separated form their parents, are housed in Texas' tent city Reuters Undocumented immigration across the US-Mexico border A two-year-old Honduran asylum seeker cries as her mother is searched and detained near the US-Mexico border Getty Undocumented immigration across the US-Mexico border Undocumented migrants ride on the top of a freight train referred to as the beast, or La Bestia Getty Undocumented immigration across the US-Mexico border A cage inside a US Customs and Border Protection detention facility in Texas Reuters Undocumented immigration across the US-Mexico border US Border Patrol Academy All new agents must complete a months-long training course at the New Mexico facility before assuming their posts at Border Patrol stations, mostly along the US-Mexico border Getty Undocumented immigration across the US-Mexico border US-Mexico border fence A group of young men walk along the Mexican side of the US-Mexico border fence in a remote area of the Sonoran Desert Getty Undocumented immigration across the US-Mexico border US-Mexico border fence in the US Man looks through US-Mexico border fence into the US in Tijuana, Mexico Getty Undocumented immigration across the US-Mexico border US-Mexico border fence US Border Patrol agent Sal De Leon stands near a section of the US-Mexico border fence while stopping on patrol on in La Joya, Texas Getty Undocumented immigration across the US-Mexico border US Border Patrol Academy US Border Patrol instructor yells at trainees after their initial arrival to the academy Getty Undocumented immigration across the US-Mexico border Memorial service in Guatemala Families attend a memorial service for two boys who were kidnapped and killed in San Juan Sacatepequez, Guatemala. Crime drives emigration from Guatemala to the United States, as families seek refuge from the danger Getty Undocumented immigration across the US-Mexico border Arrests on the border Undocumented immigrants comfort each other after being caught by Border Patrol agents near the US-Mexico border Getty Undocumented immigration across the US-Mexico border Detention holding facility A boy from Honduras watches a movie at a detention facility run by the US Border Patrol Getty Undocumented immigration across the US-Mexico border Mexican farm workers Mexican migrant workers harvest organic parsley at Grant Family Farms in Wellington, Colorado Getty Undocumented immigration across the US-Mexico border Mexican family in Arizona A Mexican immigrant family sits in the living room of their rented home in Tuscon, Arizona. The family that Arizona's new tough immigrant law had created a climate of fear in the immigrant community. Getty

Mr Trump ordered the Defence Department to aid in the housing of migrant families this week, after facing backlash for his "zero tolerance" policy that resulted in the separation of more than 2,000 migrant children from their families at the border.

Responding to the public outcry, Mr Trump signed an executive order requiring families to be kept together while facing prosecution for illegal border crossings. He also directed the Secretary of Defence to take “all legally available measures” to identify existing facilities to house these families, and to construct such facilities if necessary.