Cleaners, security guards and other support staff could go on strike next month in a row over terms and conditions at the department for Business Energy and Industrial Strategy (BEIS).
PCS which represents 92 contracted out staff at BEIS is asking workers whether they want to take strike action in July.
Members are demanding an end to low pay and improved working conditions, extra bonuses for working through lockdown and getting back annual leave entitlement owed from last year.
PCS also has concerns around Covid safety of staff and is calling on ISS and BEIS to agree a return to work protocol with the union.
The ballot will open today and closes 29 June.
PCS general secretary Mark Serwotka said: "ISS is treating its staff appallingly and despite repeated strike actions in recent years, it seems the contractor has not learnt its lesson.
"Support staff are poorly paid and the least they deserve is a decent rise and to be treated with dignity at work.
Mr Serwotka also pointed out it was the "height of hypocrisy" that in the week BEIS announces a new so-called "workers rights watchdog" that it should allow a contractor operating within the department to treat its staff so badly.
He added: "BEIS holds ultimate responsibility for the ISS contract and it is the height of hypocrisy to say you are committed to workers rights' when you turn a blind eye to bad practices by bad bosses within your own department.
"This latest episode shows why ISS must be stripped of their contract and that all staff should be taken in-house and directly employed by the civil service."