Trade union Unite said in a statement that the polling will continue for three weeks until 11 October.
The workers have raised the issues of a ‘real terms pay cut and outsourcing threats’ made by United Airlines.
According to Unite, the company offered a 5% pay rise along with an additional payment this year, as well as a 4% pay increase next year, even though it had registered a quarterly profit of $329m.
Unite general secretary Sharon Graham said: “United Airlines is offering workers a pay cut when it can afford to pay its staff fairly.
“To make matters worse, bosses want to tear up a long-standing agreement barring the use of outsourced workers.
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“These are red lines the workers won’t cross so the airline faces the prospect of steadfast industrial action.
“Unite members have their union’s complete backing to protect their pay, terms and conditions.”
Unite represents nearly 300 workers in operations, baggage handling and customer services at Heathrow.
The union said that industrial action could disrupt the operations of as many as 18 flights from the airport a day.
Unite regional officer Clare Keogh said: “Unite has a long-standing commitment from United Airlines to prioritise direct employment.
“Now management want to open the floodgates to agency and outsourced labour. Our members want fair pay and they won’t allow a race to the bottom.
“The workforce feel they have no other choice except to begin an industrial action ballot. It’s time for United Airlines to think again.”
United Airlines is reported to be holding talks with Unite with the aim of striking a deal, according to a company statement.