The move could lead to lengthy queues at airports and ports over the festive period, but the union has made clear that it will not take any strike action that would jeopardise national security.
It means that security checks will continue at UK borders but they may take much longer than normal and could also mean police or military officers are called up to handle the Channel migrant crisis.
What has the union said?
UK Border Force workers are demanding an 8% pay rise and have already rejected an offer of a 3% increase from the Home Office.
ISU general secretary Mark Gribbin said the union was considering a “longer period” of industrial action over the Christmas and New Year period when millions of people may travel abroad to visit relatives and friends.
Mr Gribbin said the strikes would also be targeted at Channel migrant work and border ports, which are key to the importation of Christmas goods and food supplies.
Tony Smith, a former Border Force director general, warned there was a risk that national security could be compromised by the strike.
He told The Telegraph: “It is very worrying. If the Border Force cutter crews come out on strike, you could potentially stop meeting the small boats in the Channel. If you are not able to stop them at sea, they will be arriving on the beaches.”
Matthew Scott, the Police and Crime Commissioner for Kent, is seeking an urgent meeting with Cabinet Office ministers and officials in order to discuss contingency plans, including whether extra officers will be needed to help with the migrants.
When does the ballot close?
The strike ballot is due to close on 31 October and more than 50% of the members have to vote as it is a key public service. To get a mandate to strike, more than 40% of the membership also have to vote in favour of industrial action.
It seems likely to go ahead as a previous indicative ballot showed that members are expected to back the move to strike.
ISU members have only taken strike action twice before, in 1984 and then 2012 over pay and pensions.
Mr Gribbin said: “We are deeply saddened and frustrated that things have reached this point. We have offered the Home Office every opportunity to enter into meaningful dialogue. They have repeatedly refused to engage with our eight per cent pay claim.
“The Home Office position is intransigent, unreasonable and disrespectful. We have now exhausted all options short of industrial action to carry forward our pay campaign.”
He added: “There will be a focus on our border ports and on Channel migrant work. We are also considering a longer period of industrial action over the Christmas and New Year period.
“We are prepared also to look at more focussed action, directed either at refraining from specific workplace activities or perhaps shorter strike stoppages, where lengthier action would risk especially severe national security consequences.”
The threatened action comes as members of the Public and Commercial Services Union (PCS), which represents more than 150,000 civil servants including some Border Force and immigration officers, is also balloting its members. Its postal vote is due to close on 7 November and is due to be announced on 10 November.
A Home Office spokesman said: “We work closely with all UK ports and airports to ensure passengers and goods can cross the border as smoothly as possible, and will do so in the event of any industrial action.
“Government have robust plans in place to deploy officers flexibly to support the flow of passengers and goods at the UK border.”