British Airways, which is being investigated over alleged cartel activities, faces yet further upheaval this summer after pilots at its Spanish partner, Iberia, voted to strike next month.
The move will affect almost a third of BA flights between the two countries during one of the busiest periods of the year.
Under a code-sharing agreement, Iberia operates one in three of the flights between the UK and Spain sold by BA. It could prove a logistical nightmare for BA as it tries to squeeze thousands of holidaymakers into free seats on other flights or send them to their destinations by alternative routes.
A BA spokeswoman said: "We are working on contingency plans to transfer people on to other flights and accommodate them on other services."
The vote to strike by Spain's Sepla pilots' union comes on the heels of the opening of criminal and civil investigations by UK and US authorities into possible collusion between BA and its rivals to set the fuel surcharges they add to the cost of tickets. BA's Waterside headquarters were raided by the authorities last week.
"There is no such thing as good timing for any type of problem," said an analyst speaking on condition of anonymity. "There are obviously going to be fewer free seats than at other times in the year."
The strike is set for the week of 10 to 16 July, when tens of thousands flood out of the UK to destinations such as Ibiza, Majorca and Barcelona.
Spain is one of the airline's premier destinations. BA holds a 9 per cent stake in Iberia and the two carriers code-share on more than 65 domestic and international routes.
The Spanish pilots called the strike to protest at Iberia's plans to start a new, low-cost airline called Catair, which is set to begin operating out of Barcelona in October. Iberia has failed to guarantee that pilots will keep their jobs when planes and routes are transferred to the new company.
Of the 49 daily round trips between the UK and Spain, Iberia operates 15, including seven out of 12 to Madrid, and half of the eight to Barcelona. Flights to destinations such as La Coruna and Bilbao could be particularly hard-hit, as they are operated solely by Iberia, which flies only once a day. BA is hoping that Iberia will be able to head off the dispute.