Around 600 more security workers, employed by Heathrow at Terminal 3, returned an 85% "yes" vote for action last week.

They will join 1,400 colleagues employed at Heathrow Terminal 5 and within campus security at the airport, who on Thursday (25 May) embarked on the first of three days’ action.

Unite general secretary Sharon Graham said the gap between Heathrow’s position and that of Unite members at the airport was widening, warning the dispute would "inevitably" lead to disruption, delays and cancellations this summer.

"It is now time for them to come back to the negotiating table and deal with this issue before further escalation occurs," said Graham. "Strike action is now set to escalate throughout the summer and Unite will leave no stone unturned in support for our members involved in the dispute."

The union claims Heathrow workers’ pay has gone down by almost a quarter in real terms since 2017. It is poised to announce further strike action "in the coming days".