British Airways cabin crew will strike for seven days later this month.
Union officials said its members made the decision after failing to reach agreement in a long-running dispute over pay and working conditions.
They will strike for three days from 20 March and for four days from 27 March.
By law, Unite had until next Monday to announce actions, giving seven days notice of any walkout.
‘This will probably cost BA something in the region of £20m (€22.22m) a day but maybe less since they have some contingency plans in place,’ said Davy Stockbrokers aviation analyst Stephen Furlong.
BA has trained staff from other areas of the company to fill-in as cabin crew during the strike and has said it will hire 23 fully-crewed planes from charter companies to help run flights from Heathrow.
The union's 13,000 cabin crew members have twice voted for industrial action as part of a dispute over job losses and changes to working practices, but a 12-day Christmas strike was halted following a successful legal challenge from BA.
BA crew have decided not to strike over the Easter holidays, which are spread across the first week of April, because of a public backlash over the planned duration of its December strike.
The airline said it was 'extremely disappointed' by the decision to strike and added that the action would cause 'massive disruption'.
'Unite's action has no shred of justification,' said a spokesman.
