Volunteers could be used to guard some of the UK's borders, the Home Office has said.
The plans under consideration are for "Border Force Special Volunteers" to be used at small air and sea ports.
A Home Office spokesman said it was "considering the potential benefits" and looking at how they are used by police forces.
But Conservative MP Charlie Elphicke told the Mail on Sunday: "We can't have a Dad's Army-type of set-up."
The Public and Commercial Services (PCS) union, which represents many Border Force staff, said the government was risking the country's security "on the cheap".
The proposal comes after recent reports have raised concerns over "poor" coverage of some minor harbours and landing places.
The Border Force carries out immigration and customs controls for people and goods entering the UK.
But an assessment by the Chief Inspector of Borders and Immigration, published in July, said there were long periods of non-attendance by Border Force at some locations as well as a shortage of staff able to use specialist scanners.
It looked at 62 normally unmanned ports on the east coast and found that Border Force officers had not visited 27 of the sites between April 2015 to June 2016.
Chief Inspector David Bolt's report also revealed the number of clandestine migrants detected at the ports had almost doubled in 12 months rising from 233 to 423.
Another report by David Anderson QC, former independent reviewer of terrorism legislation, said there were similar issues with smaller south and east coast ports, marinas and landing places, adding it was "conceivable" they might be an option for returning foreign fighters or other terrorists.
The Home Office said if it was to introduce volunteers, they would be used to "bolster" Border Force staffing levels and would not be used by Immigration Enforcement.
But Mark Serwotka, general secretary of the Public and Commercial Services (PCS) union, called the plan "ridiculous".
"Border Force are already using poorly trained seasonal workers at most ports and airports, not just at peak periods but throughout the year because of permanent staff cuts," he said.
"The plans to use volunteer Border Force specials is a further move towards casualisation of the workforce."
Mr Elphicke, MP for Dover, said he would "urge great caution before seeking to adopt a model like that used by the police, with special constables".
"Border security is a skilled job, which takes many years of training."