In a row over airport fees, Ryanair is threatening millions of passengers who have already booked and paid for their flights to Spain with an extra fee upon departure to be allowed to board.
The airline sent emails to customers booked to fly from Spanish airports warning them of the backdated fees.
'We may be forced to debit passengers for any government imposed increases in airport charges prior to your travel date', the message says.
'If any such tax, fee or charge is introduced or increased after your reservation has been made you will be obliged to pay it (or any increase) prior to departure.'
Chief executive Michael O'Leary said the warning came after the Spanish government revealed its plans to double the fees at its two main airports in Barcelona and Madrid.
Mr O'Leary said: 'You can of course reject that additional payment, cancel your flight and then not fly with us if you so wish. But we're not going to be funding the Spanish government's taxes.'
British Airways, which flies from London to Madrid, said it would absorb any increase.
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