Workers at six UK airports owned by BAA have voted three-to-one in favour of strike action in a dispute over pay.
BAA said Heathrow, Stansted, Glasgow, Southampton, Edinburgh and Aberdeen airports would have to close on any strike days, as vital staff including firefighters are due to take part.
The airports operator said the 50% turnout meant the union did not have a clear mandate for action.
Strike dates are expected to be announced next week.
BAA said: “We regret the uncertainty this vote has already caused our passengers and airline customers.
“Fewer than half of those people eligible to vote have done so and we do not believe this result provides a clear mandate for strike action.”
The Unite union, which is fighting a “measly” 1% pay offer, said 74.1% of the 3,054 staff who voted had said yes to strike action. A total of 6,185 staff were balloted.
In addition to the firefighters, the vote was also put to security officers, engineers and workers in various support roles.
Earlier, the much smaller Prospect union, which represents about 100 managerial and technical staff, said its members had also voted in favour of industrial action.
Both sides said they hoped to be able to avert the action through negotiation.
The union said it would be meeting on Monday to discuss a plan of action, leaving the weekend open for talks.
It would have to give a minimum of a week’s notice for strike action, meaning walkouts could begin from the week starting 23 August.
Some analysts suggest Unite may target the August Bank Holiday weekend that starts on Saturday, 28 August, but its leaders refused to confirm that.
Unite’s Brendan Gold said it was not their intention to ruin people’s summer holidays.
“The advice we would give to passengers is to put pressure on the company to come to reach a negotiated settlement,” he told a news conference.
No strike action has previously completely closed BAA’s airports. In the last planned walkout in 2002, action was called off at the last moment when an improved offer was made by the company.
The union says BAA staff accepted a pay freeze in 2009 and has described the offer for this year – a 1% increase, with the possibility of an extra 0.5% if the union agrees to changes to the company’s sickness agreement – as “measly” and “nothing short of confrontational”.
BAA counters that its offer is reasonable “at a time when BAA and its airline customers are seeing a decline in passengers due to the impacts of recession and volcanic ash”.
Unite is also calling for staff to receive a £450 bonus, which had been promised to them if the company had met a certain earnings target.
BAA, which is owned by Spain’s Ferrovial, missed the target by 3%. However, Unite says staff deserve financial recognition for coming so close.
The company has also said there will be no additional summer bonus this year, which is usually paid if BAA makes a profit, and is worth about £700.
Unite is currently also in a dispute with British Airways, which has resulted in 22 days of strikes so far this year.
The union has threatened to ballot for further strikes at the airline starting in September.